I tried Alien Arena a while back.
Unlike Tremulous, it also has several single player campaigns. They aren't very good, but they are there. There is also a multiplayer mode which features vehicles.
But really, the last time I tried it, it really sucked. It ran like crap, there were hardly any players, and there was some really horrible music you couldn't turn off.
And the weapon balance was totally bizarre. It has the basic traditional FPS weapon set, with a couple really weird extras , and with some of the weirdest balance I've ever seen. The railgun is a pea shooter. The rockets are slower than Lucy projectiles. The minigun is suitable for sniping all the way across the map. The BFG equivalent behaves kind of like a shotgun.
And the last time I checked, the models all had really cheesy animation.
