A rare drop, [Time-Lost Figurine], has a small chance to drop from Terokk that will allows you to transform into an Arakkoa anywhere lasting for 5 minutes.
Eating [Magic Eater], which can only be fished up from the sewers of Dalaran, has a small chance to turn you into a basilisk for 60 seconds. This effect also prevents spell casting.
Upon starting the quest [20] What We Don't Know..., you are transformed into a blood elf for 10 minutes. The disguise also causes you to speak in Thalassian and persists even after the quest is complete.
During the quest [70] Ruse of the Ashtongue you'll gain access to [Ashtongue Cowl]. It can only be used in The Eye and is the only disguise that stays on for druids. Lasts 1 hour or until you leave the instance.
After accepting the quest [10] Totem of Vark, a furbolg spirit will transform you into a Ghost Saber. The buff also increases land speed greatly, and grants invisibility for a few minutes.
During [20] A Lost Master (2), you gain access to [Enchanted Moonstalker Cloak] that will transform you into a Moonstalker for 5 minutes, or until canceled. It can only be used in Darkshore, and the spell breaks when any action is taken.
Eating [Magic Eater], which can only be fished up from the sewers of Dalaran, has a small chance to turn you into a roach for 45 seconds. This effect also prevents spell casting.
You have a chance to be randomly sheeped while in Dalaran by Minigob Manabonk <The Mischievous Mage>. Which a sometime later you get an item in the mail from him, the [The Mischief Maker], which can then be used on someone else. Only usable in Dalaran, this lasts 30 seconds or until canceled.
Horde-only
After completing [10] Zalazane from Sen'jin Village, talking to Bom'bay, he might turn you into the following: (Repeatable and year-round)
Looting Brightly Colored Eggs during the event has a chance to turn you into a pink rabbit for 60 minutes. Prevents spell casting and will break with damage.
The item [Blossoming Branch] can also be used on players in your party to transform them into a white rabbit for 60 minutes. Prevents spell casting and will break with damage.
During the quest [63] As the Crow Flies you're given [Stormcrow Amulet], which when used will transform you into a storm crow. Can only be used next to Ysiel Windsinger. You will not have control of your actions.
Using the item [Orb of Deception] a blood elf can transform into a draenei, for a limited time (5 minutes).
Using [Overseer Disguise], that is created with 5 [Costume Scraps] (drop off mobs around Blackwing Coven), will turn you into a draenei for 3 mins. Only usable in Blackwing Coven, any action will destroy the disguise. WARNING: Do not use disguise while in an animal form, druids, as the item will be used and canceled at the same time.
Eating [Magic Eater], which can only be fished up from the sewers of Dalaran, has a small chance to turn you into an emerald whelp for 60 seconds. This effect also prevents spell casting.
Meilosh at the entrance to Timbermaw Hold in far northern Felwood will sell the [Stave of Fur and Claw] to adventurers who are Exalted with the Timbermaw Hold faction. It transforms you into a furbolg for 3 minutes (1 hour cooldown).
Alliance-Only
The quest [25] Dartol's Rod provides an item called just [Dartol's Rod] that transforms you into a furbolg until cancelled, but it's only useable within Ashenvale (10 sec cooldown). If you never do [26] A New Adornment, you can keep it.
Drinking lots of [Pygmy Oil] turns anyone into a female voodoo gnome.
Using the item [Gnomeregan Pride] an Alliance player can take the look of a Gnomeregan warrior for 30 minutes. Most actions can be performed, however may hinder a few things that don't allow a "shapeshift form" (such as the Goblin Rocket Pack). It has a 4 hour cooldown.
[Discombobulator Ray] turns the target into a Leper Gnome for 12 seconds. So if you want to be the leper, have a friend on the opposing faction blast you.
Using [Super Simian Sphere] transforms you into a gorilla. It is a world drop from WotLK raids, high instances, Storm Peaks, or Icecrown. The trinket has a 5 minute effect and a 1 hour cooldown.
Using [Underbelly Elixir] has a chance to change you and everyone around you into a human mage. Lasts for 10 minutes and only happens while in Dalaran.
Death knights only.
After accepting the quest A Meeting With Fate you'll be turned into a human member of the Scarlet Crusade, along with your deathcharger into a Swift Brown Steed. Completing the quest will remove this disguise, or if you go beyond the town it will be removed, but reapplied as soon as you reenter the town.
Having the [Ultrasafe Transporter: Gadgetzan] malfunction has a chance to change you into a Kobold very briefly (10 seconds). You won't be able to move.
Having the [Dimensional Ripper - Everlook] malfunction has a chance to change you into a Kobold very briefly (10 seconds). You won't be able to move.
Those who complete [19] Farewell, Minnow in Azshara recieve a [Faded Wizard Hat] from Sorata Firespinner. This nice, pointy had also gives you the appearance of a random Horde-race (and occasional human, gnome, or night elf) mage's apprentice (30 min cooldown).
Using the item [Murloc Costume] will allow you to wear a murloc costume. Only way to have this was to have gone to Blizzcon '07. Disguise is removed when mostly any action is taken.
During the quest [72] Surrender... Not! you are provided with [King Mrgl-Mrgl's Spare Suit] which will allow you to wear a murloc costume similar to that available from Blizzcon '07. However, this murloc costume carries a white flag of truce, instead of a Blizzcon flag.
Engineers-only
Having the [Ultrasafe Transporter: Gadgetzan] malfunction has a chance to change you into one very briefly (10 seconds). You won't be able to move.
The high-level Archaeology artifact, [Bones of Transformation], makes you look like a naga (no water-breathing or swim bonus) for 20 sec (10 min cooldown). Kinda lame actually. Added in Patch 4.0.3.
Using the [Lifegiving Seed] will change your character into a random herb model and put them in a stunned state (no movement or shapeshifting). Can't be used in combat. [Herbouflage] stays until you manually cancel it by right-clicking buff icon.
Using the item [Orb of Deception] a human can transform into an orc, for a limited time (5 minutes).
During the quest [65] Who Are They? you'll be transformed into an orc to gain information. Performing any action (other than talking), or leaving the immediate vicinity, will break the disguise. Shadowy Hunters will also be able to see through the disguise.
Using the item [Winter Veil Disguise Kit] along with a [Snowball] will transform you into a snowman. You won't be able to move, but the form lasts indefinitely, until the buff is canceled.
Using the item [Orb of Deception] a dwarf can transform into a troll, for a limited time (5 minutes).
Using the item [Darkspear Pride] a Horde player can take the look of a Darkspear warrior for 30 minutes. Most actions can be performed, however may hinder a few things that don't allow a "shapeshift form" (such as the Goblin Rocket Pack). It has a 4 hour cooldown.
Engineers-only
Having the [Ultrasafe Transporter: Gadgetzan] malfunction has a chance to change you into one very briefly (10 seconds). You won't be able to move.
Using [Underbelly Elixir] has a chance to change you into a half naked tuskarr, this will also increase your fishing skill by 50. Last for 10 minutes and only works while in Dalaran.
Using the item [Orb of Deception] a night elf can transform into an undead forsaken, for a limited time (5 minutes).
In the Lost Fleet, cursed sailors and marines can cast the debuffCurse of the Eye, which reduces physical damage by 8, and slows spell-casting by 20%, and transforms you into a transparent Undead for 2 minutes.
Using [Underbelly Elixir] has a chance to change you into a slow flying wasp, not just hovering but actually flying around like if on a flying mount. Will only happen so long as you're in the Dalaran sewers. Last for 10 minutes.
Eating [Magic Eater], which can only be fished up from the sewers of Dalaran, has a small chance to turn you into a wisp for 60 seconds. This effect also prevents spell casting.
Night elves turn into one when they die and release.
Trick or treating with Innkeepers (during Hallow's End), you have a chance to get "Trick", transforming you into a wisp for 1 hour.
Using [Rituals of the New Moon] allows you to change into either a giant White Wolf, Black Wolf, Grey Wolf or Red Wolf for 2 minutes. This can be used anywhere. This form is canceled if the book is removed from your off-hand slot. You cannot attack or cast spells in this form.
First provided for the quest [71] In Worg's Clothing, the [Worg Disguise] is used in a small series of quests so that you can communicate with Ulfang in Howling Fjord. Most actions will remove this disguise. Only usable in Howling Fjord.
While being attacked by Bloodmoon Cultist or Bloodmoon Worgen on Bloodmoon Isle in Grizzly Hills there is a chance you'll get an [Infected Worgen Bite], which stacks up to 3 times. On the third bite you'll turn into a worgen (preventing spell use) for 60 seconds. Curing this disease or waiting it out is the only way to remove it.