I stumbled upon this site a few years ago and I've been using it ever since:

http://www.exrx.net/

This site has a lot of useful information, especially in regards to weight lifting exercises. They also have a muscle directory, which lists exercises that use the muscle you select.

Muscle/Exercise Directory: http://www.exrx.net/Lists/Directory.html

I couldn't find any threads about this site, so I figured I'd share it.