CubeEngine

if your in the mood for some old school first person shooter, check out the Cube Engine its not only really good, but its FREE (and open source if you like playing with that kind of thing). the mod that turned into its own game, ActionCube, is very good weighing in at 19meg download and plays VERY GOOD on ancient hardware. full network support. you'll be up and running before you know what hit you.

The second generation, Cube 2, weighs in at 170+MB download, but still plays fine on my ancient geforce 2 MX graphics unaccelerater card. try it.. you'll like it.

one more thing, the game sports its own in game editor, so if you don't like something, hit 'E' and change it, or make your own level!