It plays almost every format you can imagine and it's free under the GNU General Public License. So go download it now. Just a tip. And for all you Windows users out there: Yes, there is a download for you to.
As soon as I get home I'll try installing another media player - Plex. A friend of mine (Mathias again) has recommended it and I suppose it's more like a media center, playing movies, music and showing photos and such. A nice thing about it is that it atomatically downloads information about the movies from IMDB and displays really cool imagery together with the information about the movie. But more about this as soon as I have had the chance to try it out.