Darkmoon Top Hat. +10% rep and exp for 1 hour. Consumed on use, Unique (20). And looks really great - a top-hat with fireworks on top.
In patch 5.2 it will be sold by Gelvas Grimegate at the Faire for 10 Darkmoon Prize Tokens, and is also rarely (~5%) contained in Darkmoon Game Prize containers, which are the new reward for the 5 Darkmoon dailies. Darkmoon Game Prize typically contain a single Darkmoon Prize Token, but have a chance of containing almost anything Gelvas Grimegate sells.
Darkmoon Top Hat are not bound, so the Unique (20) limit is currently fairly meaningless - one could just stockpile them across multiple characters. Unfortunately on the PTR (but not in Wowhead's data) the hats "require Darkmoon Faire", meaning that they all disappear once the Faire has ended. How good these hats ultimately function depends on how they are coded by time of release.
Regardless, even if Darkmoon Top Hats are only good for 7 Faire days a month, there's a decent rep (especially) bonus to be gained. The main source of Darkmoon Prize Tokens to buy these hats will be the rare dungeon drop monthly turn-ins. Those have been declining in AH value of late, so now might be a good time to stock up on those drops.