AvP2 had the most bad-ass gamemode of all - but i don't remember how it was called. Survival-something.
You had one team playing as either the marines or the corporates, the other team as aliens. Marines started out in larger numbers and had a minute to prepare. Then the aliens spawned and attacked, respawning after death. Whoever they killed, also respawned as an alien. The objective was for the human team to survive as long as possible.
Was freaking awesome, with all these random players working together, fortifying in groups in defensible positions, then waiting patiently, rifles at the ready...