VLC
http://www.videolan.org/vlc/

I just want to chime in here and praise VLC for freeing me from Codec Hell. Codecs as far as I'm concered are evil because WMP rarely tells you what codec it needs, and frequently plays just the audio or garbled video.

With VLC, it just works. It's not too pretty, but I havn't run accross a video file that it won't play yet.

Matthew