This is my personal choice.
1. Oolite. Freeware version of ELITE with OpenGL graphics and some bells knobs and whistles added. This alone would be worth getting a Pandora for IMHO
2. This one's a clone rather than a port but needs mentioning. The Bioware Infinity engine. AKA the game engine that runs the Baldurs Gate games and Planescape Torment etc. Excellent RPGs with amazing storylines and excellent isometric 3D graphics. There is already a project to get this running on Linux GemRB. It's been mentioned here before in a couple of threads.
3. Finally Quake III. Not sure if it's doable but if it is possible as I believe, then this will make me a very happy bunny indeed.
Edited by BaDToaD, 26 January 2008 - 11:17 AM.