http://www.videolan.org/vlc/download-macosx.html
VLC media player is a multi-purpose multimedia tool. It can play DVDs, VCDs or read a stream from the network. It plays DivX/MPEG-4 files (and many other files) and is unaffected by the dreaded .avi bug which can cause choppy sound when playing DivX in QuickTime Player. You can also stream content to the network or a file.