QuickTime can’t play my .avi file!
Windows users probably knows about the K-Lite Codec Pack? In the simple and elegant world of Mac OS X, we have The Perian! It’s “The Swiss Army Knife for QuickTime(tm)”. I don’t know what that means, but I do know that ever since it was created I have dumped every codecs (except for the Apple mpeg2 codec) I have in my Mac OS X.
Just install it.
Alternatively, you can also use the VLC Media Player to play most video files, and especially those that aren’t standard compliant.