UT2003 is a great shooter, and so is Serious Sam. That is, if you need a no-brainer. Otherwise, Tai's list is great. Not sure about PlaneScape yet, but I'm definitely going to try it out after all the good things I've heard about it.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram