VLC Media Player for BeOS, Microsoft's forgot the name OSes, Apple Mac OSX Leopard/Tiger, Linux 2.4 upward kernel based OSes, BSD based OSes, Solaris, Sharp Zaurus, PDA Linux, etc.
MPlayer for all use not involving Menus, whose support it lacks. In win32, *nixes.
Xine for *NIXes. Its the world's best media player platform, and has really awssome features.
Media Player Classic is a good solution for Windows DVD Playback. Just ensure that you download the Home Theter PC Mod for MPC by celtic_druid.