There are a few commercial games that support linux, UT2003 certainly has a native port (I'd imagine ut2k4 does but I don't have it). Never winter nights is a great game and runs on linux without messing around with cedega
Also have a look at
http://www.garagegames.com/pg/ for a few more games that have linux ports. ZAP is great fun, you can play 2 games before having to reconnect if you're playing as a guest.
On the free games front, although not open source. If you like 2d adventure games, have a look
www.revolution.co.uk you can download Beneath a steel sky and Lure of the temptress (oldies but very playable

and run them using
scummvm which I beleive is in portage.
If you happen to own Monkey Island 1, 2 or Curse of Monkey Island you can play those in linux using scummvm as well.
Open source wise, I've not played too many. Wesnoth always gets a mention, although it wasn't my kinda game. Theres tuxracer which is ok now and then. Theres also pingus which is great fun if you liked lemmings style games. If you have a search through the forums you'll find references to a lot of open source games.
Let us know if you find anything really good
