18. Alien Swarm [Official site] (2008)
Developer: Black Cat Games
Remember when Valve released a game for free? Not free-to-play, just free. It was called Alien Swarm, it was a standalone follow-up to a mod, and it was Valve’s first released game that wasn’t a first-person shooter. Instead Alien Swarm is a four-player co-op game in which you control a character from above as you fight swarms of… yeah. You do so as one of four classes: Medic, Officer, Special Weapons and Tech, who have distinct abilities such as hacking doors, placing turrets, and healing teammates, but who all spend most of their time popping bugs with shotguns and machineguns.
Alien Swarm is simple and around three-hours long, but it’s as well crafted as everything Valve does. That’s in large part due to the level design, which funnels you and your enemies into chokepoints, dramatic last stands, and achingly long waits for slow moving elevators.
Notes: The original Alien Swarm was released as an Unreal Tournament 2004 mod, before Valve hired the team behind the game. They made Alien Swarm in Source almost as a spare time project while the same people also worked on Left 4 Dead and Portal 2. It also prepared the Source engine for another game played from a similar perspective: Dota 2.
What else should I be playing if I like this: Alien Swarm’s SDK comes with tools for randomly generating levels, automatically plopping together pre-defined room shapes that can be built in the normal Valve Hammer Editor. So you could play lots and lots of custom built Alien Swarm campaigns, I’m saying. Or the original mod. But failing all of that, try Left 4 Dead, which has similar co-op ideas but in first-person and with funny writing.