Alien Swarm
It's good, but I rarely find any servers on it.
Anyway, I'm gonna make a few comments about games I heard in the topic.
Transformice: It's fun at first, but stuffty shamans and stupid mice ruin most of the games.
Spiral Knights: Some paid content, but that doesn't restrict anything at all. It provides hundreds of hours of fun (At least for me!) and it's pretty creative, too. I'd just stay away from the PvP.
Realm of the Mad God: Very fun, Very hard. If your character dies, he dies and that's it. It's extremely frustrating when you're about to max some stats on a character and he dies to a relatively easy boss.
Synergy: I can rarely find any good servers, but it is pretty fun.
Unturned: It's pretty fun at first, but as soon as you go on a populated PvP server, you're gonna suffer the minecraft-like death penalties. (loving snipers.)
Fistful of Frags: I've played it a LONG time ago, but I do remember enjoying my time on it. I think I'm gonna get back into this.
Hopefully my views on a few games help.
Edit: Thought I may as well toss this in there.
Planetside 2: Massive armies of individual players calls for all the co-op possible. It's even better when you're in an organized clan.
