I've gotta say BZFlag. I've been known to go on 50+hour BZ benges(sp?) only stopping for food and bathroom breaks. Let's just say that messed up my sleeping schedule. My callsign is K-RiZzLe

Well, I couldn't decide between Tux Racer, Pingus and Frozen Bubble, but finally I chose Pingus, those mini-Tuxs are so kamikazes...

Battle for Wesnoth.
Fantastic!

BZFLAG, best free tank game ever.

A native port of Grim Fandango would rock... But I think it wasn't such a popular title not even for Windows.

I think BZFlag becasue im a sucker for online multiplayer games , i hate single player games :P i play Counter-Strike on my Windows Box :(

I'll second that. Maybe this offers some hope? http://www.scummvm.org/subprojects.php
Luckily this is one of the few games that run nicely for me in wine.

As far as I know Grim Fandango and Monkey Island 4 both use a similar engine, which is not compatible with the SCUMM system (which is the base for all the other Lucas Arts' Adventure 2D titles)... It has been a while, though since I last checked the official SCUMMVM site, so maybe they've found a way to support these other titles? I'm a fan of this VM as it allows me to play such classic titles as Indiana Jones and the Fate of Atlantis, Monkey Island series up to MI 3.

Edit: This Residual project looks very interesting, I'll give it a shot with my copy of Grim Fandango!

Quake III's code was released last year around August:

http://www.shacknews.com/onearticle.x/38305

I know it's been mentioned before, where's Nexuiz?

Mahjong
 

kmahjong to be precise

Yup, I was wrong, Quake III is open source now, but there are still parts of ET that are closed source.

Nethack Falconseye!! Lost more years of my life to that game than I care to think about. Too bad its not on the list.

Bobby

Here's another uncounted vote for Cube and/or Sauerbraten.

|)
|)enji

Quake III Arena!!!!! It is open source and not here? :-)