Nathan Proudlove made this nice Lego model of Wilson from the Klei’s popular video game Don’t Starve.
Someone has put a lot of effort into an excellent Lego product idea–WWE Wrestling: The Golden Era.
Clever custom Lego by Flickr user Ochre Jelly.
Excellent custom Lego Grendizer created by Marco De Bon.
Rebrickable is a search engine for Lego parts. Search on that stray Lego piece, and Rebrickable will tell you what sets that piece was included in.