Alien Arena

Alien Arena is another classic free FPS that has been around for a while. With it's unique oldschool sci-fi movie style it stands out among more traditional military shooters. It's Humans vs Aliens in a neon-filled world of UFO's and giant space stations. Gameplay is fast and frantic, with some interesting weapons and well-designed levels. There are also vehicles and a tactical mode, though both those could be used more and work better together.
Alien Arena uses a very quick and optimized engine to push around impressive effects even on low-end systems, which is cool. Online stats tracking is another nice bonus.
I have yet to really start to like this game, but it gets better each time it's updated and it's always been a very complete and finished product, worth a try! For futuristic FPS goodness, download is here.