I've wanted to do one of these since I participated in medie's Kissin' a Fool Challenge, and in July 2007 I decided to create my own community for it with new stuff to it. In this challenge LJ community, you can win an award if people vote your fanfic as their "favorite _(fandom)_ fanfic crossover". This challenge will take place every few months, so don't worry! There will be plenty other prompts and other new additions to the fandoms already accepted.
Crossover Galore Challenge was first created on June 25, 2007 and the first challenge was issued that day. We are currently on the 14TH challenge! :)