Holy cow!!
And it's still free, and it claims to unlock single player on all the old maps (and new ones?) with improved bots...
Damn, my group and I used to love Alien Swarm. I only stopped playing because my new computer had an odd glitch with it, and we'd kinda played it to death (I "reset" thrice, which does nothing but give you badges and the thrill of leveling up again).
If someone isn't familiar, it's a top-down action shooter where you control one marine in a squad of 4. There are four classes, with two significantly different characters for each class. Like, I usually played the "purer" medic because my group kept walking into facehuggers, but there's also a medic that's better at combat.
The engineer class is occasionally required to solve repair minigames while the squad defends them, but also has stuff like an aim-assist module on the assault rifle. There's also a class with autocannons, and a leader class with shotguns which buffs or something.
It's also has a nifty mechanic for reloading, apparently cribbed from Gears of War, where you can reload a lot faster if you press the button at the right time. There's a clear visual indicator, but it can be tricky since in the situations where you want a rapid reload... you're probably dodging xenos.
Melee is also very satisfying. Particularly on normal, or if you equip a power fist or chainsaw. Yeah, it feels a lot like clearing a Space Hulk sometimes. Except the characters are quippy rather than grimdark... Usually. I still love the pure medic's "I'LL KEEL YOU ALL" when meleeing. That's what the bugs get for depleting my supplies hurting my friends...