There are actually quite a few such sites:

http://en.wikipedia.org/wiki/Wikipedia:Wiki_Game

It's one of these concepts that keeps poping up independently at many places (there used to be edit wars at Wikipedia pages about manual versions of this game with many people claiming they invented it :).

I did one myself long ago:

http://wikifighting.com

It was one of the first and it got pretty popular for a brief period of time.

The trouble with such sites is that you have to babysit them because they will get broken when Wikipedia changes.