Nexuiz (as previously mentioned) is an absolute blast. Minstagib with low
gravity, grappling hooks, and a large open map is intense.
Brutal DOOOM and FreeDOOM are an option that requires a little bit of setup,
but it's worth it.
Minetest (also mentioned) is a lot of fun with the right mods.
BZFlag is a hoot. Don't let the graphic styling fool you, it's a lot of fun.
The Bub's Brothers is a browser based (networked) game that has a lot of
old-school arcade style games in it.
I'd have to check the license on Warsow but it's another enjoyable FPS.
