"Hey, I can't open this video file in Windows Media Player" VLC Media Player! "I get a message that I don't have a DVD decoder installed when I try to play a movie" VLC Media Player! "I'm tired of having 5 media players on my computer" VLC Media Player! "I want a media player that runs on all of my systems, MAC, Windows, and Linux" VLC Media Player!
I can't say enough about this program. It plays almost everything you throw at it, and has network-streaming features, remote interface capability. There are installs available for Windows, MAC OS X, Linux, BeOS, and the code can be compiled to run on BSD, Solaris, and QNX. You can find it here.
