http://www.dvdtracks.com is a site that points to MP3 (and other audio format) files that you can download to play along with movies. This includes joke tracks, fan commentaries, and criticism. Right now, the selection mainly is for popular "geek" films: sci-fi, horror, action. They also have tips on the site about how to record your own commentary. I'm personally going to download the track for the movie "Hackers" sometime and see just how much fun they have with this.