Along the same lines as VLC, Perian allows all sorts of video formats to play in Quicktime itself, which is a little more convenient than having to launch VLC.  Perian installs as a System Preferences pane, and so has a very small footprint.  This is one of the first things I install on any new Mac.

I can’t remember the last time I had a video file that actually played correctly in Windows Media Player. That’s the reason I turned to VLC player, which will play pretty much anything you can throw at it right out of the box. If you want to find out more, read on after the jump.