Has someone ever sent you an attachment in an email that is supposed to be a video file but it will not play with Window’s Media Player, Apple’s Quicktime, or Real Player?  Then I recommend the VideoLAN project.

I you have a passel of MPEG-1/2/4 files, DivX files, even DVDs, or live video on your computer or home network and you want to play them, VideoLAN's VLC player will do the trick, whether you are running Linux, OS X, or Windows.

Check out this diagram for a nice illustration of your options.

I use  VLC to watch the files that I am able to download off my ReplayTV DVR.  It allows me to watch in a small window, like a picture in picture on the computer or at full screen mode.

Best of all, it is completely free.  That is a great value at twice the price!