Transformice, but it was pretty popular over on /v/ for a while (4chan). Apparently the creators are Bronies! You can use the cheese dollars (collected by playing the game to deck out your characters in pony hair.
One more step to internet domination, good work everypony!
Thanks to Xen for the heads up
Game link is HERE!
Note: I was told to tell you not to raid their servers! I doubt any of you will listen though!