Valve have released ALIEN SWARM, a free co-op shooter for up to 4 players (there is an offline single-player practice mode but no single-player campaign). It's basically a remake of the old, brilliant ALIEN BREED games, a top-down shooter where four marines have to fight their way through an alien-infested base, retrieve vital data files, blow stuff up, and escape.

The trick here is that, inspired by LEFT 4 DEAD, each of the 4 players has different weapons, skills and equipment, and combining them is necessary to escape (for example, the party needs one welder and one area-healing device to make it through any given level). You can also pick up additional equipment like smart guns (those proximity-triggered auto-cannon things from ALIENS) to help out.

It's brilliant fun, especially when the alien swarms become insanely huge and start overrunning the party and ammo gets low, leading to screams about friendly fire and having to resort to melee combat with creatures with more teeth than the entire Osmond family