These kids in France have written an entire cross-platform video streaming solution that supports MPEG, MPEG-2, MPEG-4, DivX, mp3, ogg, and DVD. Stream video live from one system to another, regardless of platform, or just use the client to view files locally. It supports Windows, Linux, iPaq Linux, BeOS, Solaris, NetBSD, FreeBSD, etc., etc., and it's open source. Best of all, there's a client for the Mac written in Cocoa, and DivX movies play flawlessly-- no avi2mov or DivX Doctor conversion required.
Why are you still reading this?!?! Go try out VideoLAN Client right now!