Alien Swarm is a free co-op tactical shooter currently available on steam, here: http://store.steampowered.com/app/630/

There's absolutely no good reason why you don't all have this already.

The KTX L4D server's up; you should see it when you launch the game. You can also connect directly to 8.12.20.46:27015

We've decided that in group matches, IE, if more than 2 or 3 of us are in the same game, tag up as [KTX]Name. Not strictly enforced, but if there's six of us in a game, there's no reason not to change your name for the duration of the playtime.