Quake 2 may be old, but it’s still played by many people. Id Software kindly open sourced the code last year, and here’s the result: a new version of Quake 2 with many bugfixes and new features, and more to come.
I could never get past The Big Gun without Q2 crashing. Now I can get back to the game, and with full OpenGL acceleration under Linux too. <a href="http://quakeforge.net/">QuakeForge</a> also have an enhanced version of the original Quake. Link: <a href="http://quakeforge.net">QuakeForge</a>