I recomend warsow, kinda looks a bit like tron.

Quakelive is free and you play right in the browser.

Digital paintball is an old favourite. it uses the quake 2 engine and basically, it's paintball. You have teamdeath match, attack and defend and capture the flag.

wolfenstein enemy territory is good for your ww2 craving.

Lastly is alien swarm, but you need a mordern CP to run it well. Although you do start with an overhead view, you can switch to a first person view in the console making blowing aliens to bits, all the more fun.