Ashisogi Jizou (Beast-Killing Jizou)
Owner: Kurotsuchi Mayuri, Captain of the 12th Division
Command Phrases: "Kakimushire!" ("Tear off!")
Abilities: When in its released form, Ashisogi Jizou's three blades secrete a substance that interrupts the nerve signals governing movement, effectively paralyzing any part of the body they injure. This substance is highly specific in its effects and does not affect any other types of nerve activity, meaning limbs it paralyzes can still feel pain. Mayuri can also transform his body into liquid by stabbing himself with Ashisogi Jizou. This ability is used as a last resort measure to escape battles when Mayuri is losing and probably can't be used on others.
Bankai Name: Konjiki Ashisogi Jizou
Bankai Abilities: In its Bankai form, Ashisogi Jizou becomes an immense sluglike creature with a stomach covered by a large number of blades. It also emits a potent and eventually fatal toxic vapor which is derived from Mayuri's blood.

Benihime (Crimson Princess)
Owner: Urahara Kisuke, former Captain of the 12th Division
Command Phrases: "Okiro!" ("Wake up!") and "Nake!" ("Sing!")
Abilities: Benihime's sealed form is that of a blade hidden in Kisuke's cane instead of a normal katana. Benihime's activated abilities haven't been really described in detail yet, but it seems to be able to shape and manipulate blood--Kisuke creates a barrier from his own blood to shield himself from an attack from Ichigo, and it appears he uses Benihime to do this.
Bankai Name: Unknown
Bankai Abilities: Unknown

Fujikujaku (Wisteria Peacock)
Owner: Ayasegawa Yumichika, 5th Seat of the 11th Division
Command Phrases: Unknown
Abilities: Fujikujaku's released form is a curved, scythelike weapon that can split into a fan of five blades. Yumichika pretends that this is his sword's only ability, concealing the fact that Fujikujaku is actually capable of draining spiritual energies from his opponents, which Yumichika can apparently use to heal his own injuries.
Bankai Name: n/a
Bankai Abilities: n/a

Gegetsuburi (Five-Form Head)
Owner: Oomaeda Marechiyo, Vice Captain of the 2nd Division
Command Phrases: "Buttsubuse!" ("Smash!")
Abilities: Gegetsuburi's rather unsubtle activated form is that of a big spiked ball on a chain. Ichigo kicks Oomaeda's ass before we can see if it does anything else.
Bankai Name: n/a
Bankai Abilities: n/a

Gonryoumaru (Stern Spirit)
Owner: Unnamed Vice Captain of the 1st Division
Command Phrases: "Ugate!" ("Pierce!")
Abilities: Gonryoumaru's activated form resembles a Western fencing blade (a saber, I think?). As with Gegetsuburi, Ichigo kicks its owner's ass before its abilities can be shown in any detail.
Bankai Name: n/a
Bankai Abilities: n/a

Haineko (Ash Cat)
Owner: Matsumoto Rangiku, Vice Captain of the 10th Division
Command Phrases: "Unare!" ("Growl!")
Abilities: Haineko's blade seems to transform into a powdery cloudlike form--probably ash, given the name. What it actually does in this form has yet to be shown.
Bankai Name: n/a
Bankai Abilities: n/a

Hoozukimaru (Ogre Lamp)
Owner: Madarame Ikkaku, 3rd Seat of the 11th Division
Command Phrases: "Nobiro!" ("Grow!") and "Sakero!" ("Split!")
Abilities: Hoozukimaru transforms from a katana into a spear. The spear, in turn, is capable of splitting into a three-sectioned weapon joined by lengths of chain.
Bankai Name: n/a
Bankai Abilities: n/a

Hyourinmaru (Ice Ring)
Owner: Hitsugaya Toushirou, Captain of the 10th Division
Command Phrases: "Souten ni zase!" ("Sit in the frozen sky!")
Abilities: Hyourinmaru's activated form resembles a normal katana, but it sprouts a length of chain from the hilt that Hitsugaya can use for entangling attacks. More importantly, this chain also seems to be used to direct the effects of Hyourinmaru's main ability: the power to create and shape huge volumes of ice and freezing water.
Bankai Name: Daiguren Hyourinmaru (Hyourinmaru of the Great Frozen Hell--much Buddhist term knowing love to [info]pocketmofo)
Bankai Abilities: Unknown; Aizen defeats Hitsugaya before Hitsugaya can really do anything with his Bankai. The name refers to one of the eight cold Buddhist Hells, a places where the skin freezes so badly it rips and the flowing blood resembles red lotus flowers (the "guren" part contains the kanji for "crimson/deep red" and "lotus."

Itegumo (Frozen Cloud)
Owner: Kotetsu Isane, Vice Captain of the 4th Division
Command Phrases: "Hashire!" ("Run!")
Abilities: Much like the activated form of Fujikujaku, Itegumo's blade splits into a three-pronged form, sort of like a trident. But, as per Gegetsuburi and Gonryoumaru, Itegumo's owner is taken down too quickly for its abilities to be shown.
Bankai Name: n/a
Bankai Abilities: n/a

Katen Kyoukotsu (Flower Heaven Lunatic Bone)
Owner: Kyouraku Shunsui, Captain of the 8th Division
Command Phrases: "Hanakaze midarete, kashin naki. Tenpuu midarete, Tenma warau!" ("The flower wind is disturbed, the god of flowers weeps. The wind of Heaven is disturbed, Tenma laughs!" --thanks to [info]pocketmofo)
Abilities: Katen Kyoukotsu is one of the two Zanpaku in Soul Society history which consists of two swords that are counted as a single weapon. In its activated form, Katen Kyoukotsu's blades become these big, elaborate sort of cleaver-looking things, but its actual abilities have yet to be shown.
Bankai Name: Unknown
Bankai Abilities: Unknown

Kyouka Suigetsu (Mirror Flower Water Moon)
Owner: Aizen Sousuke, Captain of the 5th Division and Total Bastard
Command Phrases: "Kudakero!" ("Shatter!")
Abilities: Aizen describes Kyouka Suigetu's Shikai ability as "complete hypnosis." Essentially, anyone who sees his sword's activation falls under its spell. Any time thereafter, Aizen can use this ability to create elaborate illusions that fool all five senses of anyone who has witnessed Kyouka Suigetu's activation in the past. This power has no effect on the blind. Also, as [info]pocketmofo tells us, the name of the sword works out to a phrase (go go four-kanji compounds) that means "like a flower reflected in a mirror or the moon's image reflected on the water" and refers to something that can be seen but not grasped.
Bankai Name: Unknown
Bankai Abilities: Unknown

Minazuki (This name is untranslatable. Absolutely. If you want to take a crack at coming up with a coherent translation, the kanji in it are: mi [meat/flesh], na [water droplets/dew], zuki [suck blood/sound of a shoal of fish or flock of birds feeding]. Best of luck, and many thanks to the valiant efforts of [info]rukia, [info]lacewood, [info]stanking, and [info]xerosphere for helping me find what the last two kanji meant in the first place! Edit: [info]the_cowch has suggested "Fleshdrinker." I like the sound of that.)
Owner: Unohana Retsu, Captain of the 4th Division
Command Phrases: Unknown
Abilities: Minazuki's Shikai form is this big freaky creature that looks like it was brought into being by taking a cyclops, an alien, a stingray, and a whale, and shoving them all in a blender. It can fly, and it can also carry things in its stomach. Injured people placed in Minazuki's stomach will heal faster.
Bankai Name: Unknown
Bankai Abilities: Unknown

Nejibana (Screw Flower)
Owner: Shiba Kaien, late Vice Captain of the 13th Division
Command Phrases: "Suiten sakamake!" ("Uncoil the sky and water!" - tentative translation)
Abilities: Unknown. Nejibana is destroyed by a Hollow before its activated form can even be shown.
Bankai Name: n/a
Bankai Abilities: n/a

Ryuujin Jakka (Flowing Blade Like Flame--thanks to [info]lacewood for this)
Owner: Yamamoto Genryuusai Shigekuni, Captain of the 1st Division and Captain-General of the Shinigami
Command Phrases: "Banshou issai kaijin to nase!" ("Turn all creation to ash!")
Abilities: Ryuujin Jakka is just plain unfair. In its activated form, its blade is engulfed in an aura of fire. Anything the blade passes over or touches is reduced to ashes--it's just that goddamn hot. When Yamamoto first fires the thing up, the mere act of waving it around a few times soon has the surrounding area completely consumed by flames.
Bankai Name: Unknown
Bankai Abilities: Unknown but probably scary

Senbonzakura (A Thousand Cherry Trees)
Owner: Kuchiki Byakuya, Captain of the 6th Division
Command Phrases: "Chire!" ("Scatter!")
Abilities: Senbonzakura's blade splits into a cloud of tiny blades that, due to the way they absorb and reflect light, resemble scattering sakura petals. They're quite a bit more dangerous than that, though, capable of shredding opponents they are directed at.
Bankai Name: Senbonzakura Kageyoshi (A Thousand Cherry Trees - Scene of Crags)
Bankai Abilities: Senbonzakura's Bankai form is, in essence, its Shikai ability writ large; Byakuya drops his sword blade-down to the ground. It sinks into the ground as though it were falling into a pool of liquid, and then two rows of huge katana blades rise from the ground behind Byakuya. These then scatter into petal-blades as per Senbonzakura's normal ability, creating a veritable deluge of the things capable of levelling whole buildings. In this form, Byakuya can use a couple of special attacks: Senkei Senbonzakura Kageyoshi and Senbonzakura Shuukei: Hakuteiken. The former, in which the "Senkei" part means "Massacre Scene," compresses the petals of Senbonzakura Kageyoshi into a large number of swords, increasing their ability to inflict damage. Shuukei: Hakuteiken means "Final Scene: White Emperor's Sword." This is Byakuya's final technique, as the name suggests, and arranges his Bankai's petals around him in a sort of winglike shape, allowing him to perform a single huge, overwhelming cutting attack with them.

Shinsou (Divine Spear)
Owner: Ichimaru Gin, Captain of the 3rd Division
Command Phrases: "Ikorose!" ("Shoot to death!")
Abilities: Shinsou's sealed form is that of a wakizashi--a short-bladed Japanese sword. When released, its blade extends at an extremely rapid rate, essentially "shooting" the point forward like a projectile.
Bankai Name: Unknown
Bankai Abilities: Unknown

Sougyo no Kotowari (Truth of Pisces--thanks again, [info]pocketmofo!)
Owner: Ukitake Juushirou, Captain of the 13th Division
Command Phrases: "Nami kotogotoku, waga tate to nare. Ikazuchi kotogotoku, waga yaiba to nare!" ("All the waves, become my shield. All the thunder, become my blade!")
Abilities: Sougyo no Kotowari's sealed form is a single katana, but when activated, it is revealed as the other two swords, one weapon Zanpaku. Its activated form is that of two katana-like swords with a length of cord joining them at the hilt. Its abilities have yet to be shown.
Bankai Name: Unknown
Bankai Abilities: Unknown

Suzumebachi (Hornet)
Owner: Soi Fong, Captain of the 2nd Division
Command Phrases: "Jinteki shakusetsu" ("Sting all enemies to death"--thanks to [info]pocketmofo yet again)
Abilities: Suzumebachi is an assassination weapon. In released form, it becomes a needlelike weapon worn on one of Soi Fong's fingers. When this weapon inflicts an injury on an enemy, a symbol resembling a butterfly appears on them. If Soi Fong strikes a point so marked a second time, the enemy dies. In the past, these symbols would fade over time, but Soi Fong has trained the ability to completion--they can now never be erased, unless Soi Fong herself wills it.
Bankai Name: Unknown
Bankai Abilities: Unknown

Suzumushi (Pure Insect)
Owner: Tousen Kaname
Command Phrases: "Nake!" ("Sing!"--uses a different kanji than the one used in Benihime's command phrase, but the meanings are similar)
Abilities: Suzumushi has been shown to have more than one ability in Shikai form. One is to emit sound waves that can knock targets unconscious; Tousen uses this ability to subdue Ishida after Ishida's fight with Mayuri. The other is an attack called "Suzumushi Nishiki: Benihikou," or "Pure Insect Second Style: Crimson Flying Locust." This attack generates a swarm of slender blades that rain down on Tousen's target.
Bankai Name: Suzumushi Tsuishiki: Enma Koorogi (Pure Insect Final Style: Devil Cricket)
Bankai Abilities: Suzumushi's Bankai is generated from the ring attached to the sword's handguard. It undergoes a complex transformation that creates a huge barrier around a certain area. Within that barrier, anyone not in direct contact with Suzumushi is robbed of all sensory input, including supernatural senses, leaving them vulnerable to attack.

Tengen (Heaven's Punishment--corrected a bit by [info]pocketmofo)
Owner: Komamura Sajin, Captain of the 7th Division
Command Phrases: Unknown
Abilities: It gets big sometimes. Tengen's ability is apparently to create a gigantic image of itself being swung by an equally gigantic hand--oh yeah, and producing a fittingly huge swath of destruction as a result.
Bankai Name: Kokujou Tengen Myouou (Black Cord Heaven's Punishment Bright King--that's going from memory, though, I haven't been able to track down my 148 raw. It's also a complete literal translation of all the kanji; not sure how to make it into more of a phrase.)
Bankai Abilities: It gets even bigger! Kokujou Tengen Myouou is simple, a big demon thing with a big sword. We never really got to see it fight, though, as the battle in which Komamura summoned it was interrupted. According to [info]pocketmofo, who is a Golden God when it comes to these things, the "Kokujou" part refers to one of the levels of Buddhist Hell, in which thieves and murderers are bound up by black iron cords and then chopped at with cleavers. The "Myouou" part refers to a Buddhist teacher who uses force/anger/rage to guide others. Pretty appropriate, all in all.

Tobiume (Flying Plum)
Owner: Hinamori Momo, Vice Captain of the 5th Division
Command Phrases: "Hajike!" ("Snap!")
Abilities: Tobiume's released form is capable of generating fire. This can be released in the form of short-range explosive blasts or concentrated into fireballs and thrown.
Bankai Name: n/a
Bankai Abilities: n/a

Tsunzakigarasu (Splitting Crow)
Owner: Ikkanzaka Jiroubou, 4th Seat of the 7th Division
Command Phrases: "Habatakinasai!" ("Flap your wings!")
Abilities: Tsunzakigarasu's blade transforms into a number of two-bladed shuriken that swirl and dance around Jiroubou. Even if these are destroyed, more can be produced by "sheathing" the sword and drawing it again.
Bankai Name: n/a
Bankai Abilities: n/a

Wabisuke (Apology)
Owner: Kira Izuru, Vice Captain of the 3rd Division
Command Phrases: "Omote wo agero!" ("Show your face!")
Abilities: Wabisuke's released form is a hooklike weapon. Anything that Wabisuke strikes in its released form doubles in weight, and if it strikes the object again, the weight is doubled again. According to Izuru, the name "Wabisuke" itself refers to this ability; people afflicted with it tend to start to pitch forward, bringing their heads towards the ground as though they were bowing to him in apology.
Bankai Name: n/a
Bankai Abilities: n/a

Zabimaru (Serpent's Tail)

Owner: Abarai Renji, Vice Captain of the 6th Division
Command Phrases: "Hoero!" ("Howl!")
Abilities: Zabimaru transforms into a large blade divided into sections. When swung, it stretches out, becoming an enormous whip-like weapon.
Bankai Name: Hihiou Zabimaru (Baboon King Serpent's Tail)
Bankai Abilities: Hihiou Zabimaru's form is that of an enormous skeletal serpent with a wild mane and a demonic skull of a head, and its power is rather similar to that of Shikai-form Zabimaru: it lashes likes a whip. Of course, its destructive power is greatly increased, and it has the power to split at the joints to avoid attacks--the individual pieces are linked by Renji's aura, so he can reassemble them at will.

Zangetsu (Cutting Moon)

Owner: Kurosaki Ichigo, independent Shinigami
Command Phrases: Unknown (probably none)
Abilities: Zangetsu has no sealed form; it is always in its Shikai shape. Its ability is to take in Ichigo's spiritual power and use it to create an enormously powerful, long-range cutting attack called "Getsuga Tenshou," or "Lunar Fang that Pierces Heaven."
Bankai Name: Tensa Zangetsu (Heavenly Chain Cutting Moon)
Bankai Abilities: Taking a rather different philosophical tack than most Bankai forms, Tensa Zangetsu focuses on speed rather than brute offensive power. When activated, Zangetsu becomes an ordinary-sized katana with a black blade. Ichigo also gains the cool coat worn by Zangetsu's humanoid form. He is then able to move with all but unmatchable speed while still retaining a large amount of the sheer power of Zangetsu's Shikai form.