Perian - Perian is a free, open source QuickTime component that adds native support for many popular video formats. Install that and QuickTime, and hence Front Row, will play back all sorts of stuff that it doesn't out of the box such as divx.
For anything that Perian won't handle, and to play DVD's from any region without mucking around with changing your drive region (which you can only do five times) try
VLC