Problem: three crucial episodes of Six Feet Under saved as AVI files on your Mac, and the damn things won't play.
Solution? VLC media player, a free multi-operating system media player.
The breadth of platforms VLC runs on is almost as impressive as the list of audio and video formats that it plays. It's free software, so the price is right. Works on Windows, Mac OS X and just about any other operating system you can name.
VLC media player