I just got a huge craving for some coop games, and in my search for games, I remembered this gem: Alien Swarm. http://store.steampowered.com/app/630/ Maybe you've heard of it, maybe you have not. If you haven't, all you need to know it's a top-down coop shooter where you frantically run through pretty levels while shooting aliens like a madman. If you're not moving you're either managed to find a moment to replenish your ammo or busy protecting your tech from hordes of aliens while he's trying to open a blocked door. There's 4 classes, 8 characters and 4 player coop, and a pretty sizable arsenal of weapons and devices available through leveling up. It requires teamwork, it requires balls of steel, and the best part is that it is free. So, if you feel a need to shoot anything that moves(except your allies, but you'll probably blow them away by accident anyway) with your friends, this is the game for you. You can play it with less than 4 players, but it gets VERY hard even on easy difficulties. And yeah, it's 5 years old, but it's still badass as hell.
I've played a few user created campaigns, but I find that they don't hold up in level quality to the official one. But I might just have found bad ones It's too bad that Valve didn't support it for too long. Still a great game, but there could have had a lot more user created content if it had gotten some more love.
