http://xbmc.org/
I've been running the development versions of this on my Ubuntu HTPC for about a month now and am extremely impressed. It does everything it is supposed to, is very stable, and very nice looking. The linux version can do hardware-accelerated video decoding, which lets me watch my Blu-Ray rips on a 4 year-old computer.
If you like pretty, also check out the Aron skin:
http://www.aeonproject.com/gallery.html
