The Yip Yip Martians from Sesame Street were a hit at DragonCon 2011. The description at YouTube says it all:
simply put, one of the most entertaining and crowd pleasing cosplay costumes EVER... there wasn't a single person who saw these guys who didn't explode into joy.
If you are not familiar with these characters, you can watch the Muppets in one of their original Sesame Street sketches. -via reddit