Cosmetic items weren't really a part of Diablo III. Players could get a few pets and wings as rewards for spending money on a collectors' edition or on a Blizzcon ticket, and that was it. The upcoming patch 2.4.1 will change this by adding a ton of new cosmetic loot. Patch 2.4.1 won't be a huge one, but the amount of new wings, pets and other transmog stuff is remarkable. The patch's on the PTR and people have already found some really rad items, dropped by various bosses, uber bosses, treasure goblins and rare monsters.
The top one, for example, is a butterfly-looking wing, called Wings of Kokabiel, dropped by the uber version of Maghda. But here's some more. Building a character with butterfly wings, a Starcraft Terran marine helmet and a Yeti pet will be absolutely possible. Hooray!
Wings of Kokabiel (via Bluddshed)
Star helm and Star pauldrons (via Battle.net forums)
Quinquennial Sword (via Battle.net forums)
Cow King pet (via Battle.net forums)
A weird-looking Yeti pet (via Battle.net forums)
Cute pet called Galthrak (via Battle.net forums)
Crab helm called Helm of the Cranial Crustacean (via Battle.net forums)