As you all know, there is quite a variety of items ingame that change your look (And no, I'm not talking about faction/race changes, gear or the barber). Here's a guide of the ones I've been able to find .
1. Dartol's Rod of Transformation
This item transforms you into a Furbolg, and has a 3minute cooldown. It is alliance-only and is rewarded from the quest Raene's Cleansing.
2. Orb of Deception
This BoE item transforms you into a member of the opposite faction, for 5minutes, and has a 30minute cooldown. It drops many Vanilla monsters, check out the link for a list.
List of transformations:
Gnome Male <-> Tauren Male
Gnome Female <-> Tauren Female
Human Male <-> Orc Male
Human Female <-> Orc Female
Night Elf Male <-> Undead Male
Night Elf Female <-> Undead Female
Dwarf Male <-> Troll Male
Dwarf Female <-> Troll Female
Blood Elf Male <-> Draenei Male
Blood Elf Female <.> Draenei Female
(Credits to Excorsism from Wowhead)
3. Orb of the Sin'dorei
This item transforms you into a Blood Elf, for 5minutes, and has a 30minute cooldown. It drops from any of the bosses in MgT heroic, with Kael'thas having the highest chance for it.
4. Iron Boot Flask
This item with a 60 minutes cooldown turns you into an Iron Dwarf for 10minutes. It also occasionally zaps enemies you attack for a bit of damage. You can buy this for 10 relics of ulduar.
5. Frenzyheart Brew
This item, only available for people Revered with the Frenzyheart brew who buy the Jar, transforms you into a Frenzyheart. It lasts 5minutes and has a cooldown of 5minutes. It has no charges/uses.
6. Time-Lost Figurine
This trinket transforms you into an Arrakoa, and has a 30 minutes cooldown. It has an 8% dropchance from the summonable boss Terokk.
7. Savory Deviate Delight
Do I really have to explain..? Fishy that turns you into a Pirrrrate or a ninja for 1 hour.
8. Carved Ogre Idol
This item turns you into a big red angry ogre for 10 minutes and has a 10 minutes cooldown. It's obtainable from the Trading Card Game.
9. Noggenfogger Elixir
I'm not even going to explain what it does... Unless there are people living under a rock here.
10. Pygmy Oil
This item doesn't transform you right away: first every stack will make you smaller, but if you drink enough you'll turn into a pygmy! This is a female gnome with a witchdoctors mask. The Oil is obtainable from alchemists.
11. Rituals of the New Moon
This item created by Scribes transforms you into a wolf, and has a 10 minutes cooldown. It is an offhand item.
12. Gordok Ogre Suit
This item transforms you into an ogre for 10minutes, and is used usually in Dire Maul. It is made by Tailors or Leatherworkers.
13. Deathbringer's Will
This item transforms you in one of the Northrend races. The race also decides what stat you will get. The buff/transform lasts for 30seconds.
14. Underbelly Elixir
This item can be found throughout the whole Underbelly of Dalaran. It can has various effects, for example everyone can change into a mage for 10minutes, or transform you into a (flying) wasp or tuskarr!
15. Mojo
If you /kiss this pet, you will change into a frog for some seconds. The pet is obtained from de-hexxing frogs in Zul'aman. The sticks to de-hex drop from various mobs there.
16. Super Simian Sphere
This is a world drop item from Northrend mobs, and turns you into a monkey with a big pink circle around you. It lasts for 5 minutes and has a 1 hour cooldown.
17. Magic Eater
This fish can be fished up in Dalaran, and has a chance to turn you into a basilisk for 60 seconds. Note: don't use this in raids, since it's a debuff (can't rightclick it off) and prevents spellcasting.
Edit: Also turns you into a wisp, a dragon, may give well fed buff etc. etc.. [Credits to Echoes, thanks]
18. Hallowed Wand - Bat and all other wands.
These wands are obtained by various ways during the Halloween event in World of Warcraft. You can use them to change a partymember into the animal/person the wand is for. The wands got 5 charges.
19. Brightly Colored Egg.
These eggs spawn only during the Noblegarden event, and looting them gives a chance to change you into a bunny!
20. The Mischief Maker
This item gets sent to you in mail when a stupid gnome in Dalaran polymorphs you. It gives you the chance to be a retard as well and change a random person in dalaran into a sheep for 15 seconds!
21. Bom'bay
After you've completed the quest "Zalazane", this troll in Sen'jin village gives you the option to let him cast spells on you. With these, you can (together with other things) change into a Chicken, frog or snake
22. Turkey Shooter
This item is only available during the Pilgrim's Bounty event, and enables you to shoot another player and turn them into a turkey for 1 hour. It's a reward from various quests.
23. Gnomish Poultryizer
This engineering-only item changes your target into a chicken for 15 seconds (if it doesn't malfunction), and has a 5minutes cooldown.
24. Dire Brew
Dire Brew can be looted in the Grim Guzzler in BRD, after making the npc at the fire pass out, and changes your appearance to an Iron Dwarf for 60 minutes.
25. Hook of the Master Angler
This item changes you into a fish (only when swimming, duh..) for an infinite time, giving you water-breathing and increased swimming speed. This is a reward from winning the STV fishing tournament.
26. During Winter's Veil, you can walk through one of the transporters located throughout Azeroth to turn you into a red or green Gnome.
27. Discombulator Ray
This item, made by engineers, changes your target into a Leper Gnome for 12 seconds. This is only useable on enemies, thus you'll have to ask someone of the opposite faction, or go duel someone.
28. Hallow's End Pumpkin Treat
This item, obtainable during the Hallow's End event, gives you a chance to turn into a pirate when eaten (other effects: get bigger/orange/ghostly).
29. Murloc Costume
This item was given away to participants of Blizzcon 2007, and is thus no longer obtainable. It transforms you into a murloc, has a casttime but no cooldown, and lasts till you perform an action like mounting or casting.
30. Winter Veil Disguise Kit
This item transforms you into a stationary Snowman, which last forever. It requires a snowball to use, and the item is only obtainable during the winter's veil event.
31. If you completed the questline in Stormpeaks, every time you enter the Hyldnir area's you turn into a big blue ugly vrykul.
I'm sure I forgot some, so please just reply if you see any of those. However, remember that I left out most zone-specific items/transforms, as well as class-specific ones like druid forms.
-Aldun