Can you believe it's been 9 years since the release of Wolfenstein 3D? This id Software game changed gaming, that's for sure. Also the engine, one of the many impressive game engines by John Carmack. If you're feeling nostalgic you can download the 836k shareware episode. There is also that OpenGL port of the original game if you want to run the game under Windows with prettier graphics.
Of course, we also have the new Return to Castle Wolfenstein coming later this year. Though it looks to be different with several weird monsters instead of all Nazi soldiers and a couple of weapons not really available during the second World War, it should hopefully still be another good game starring William J. "B.J." Blazkowicz.