Assault Cube is an open source 3d shooter done in the old school deathmatch style of gameplay, but with much newer graphics. Gameplay is especially good in this game due to the way it's implemented. Unlike the original Cube and Cube 2 on which it's built, Action Cube provides you with a much better and fuller gaming experience.
I've found that it runs much smoother, has zero lag, and the gameplay is quite a bit more fun and challenging. It's like mixing what made the old school FPS games fun with a lot of what makes modern games, little counterstrike, fun as well.