Tremulous.

Tremulous.
Pretty fun First Person Shooter, based and run off the Quake Engine, (As is UrT), and includes more classes.
Game type is TEAM based / Kind of deathmatch style.
OBJECTIVE is basically kill enemies; go up in stages; destroy enemy base; kill remaining enemies; WIN.
Simple game role.
Here's a video of me playing Trem back around last weekend. I was doing good until the point the video was shot. Then I'd failed miserably and started not getting any kills at all.

that is a fun game