The game folks over at Valve (the creators of games such as team fortress 2, portal, halflife series, left 4 dead series) have released for free on their Steam system, Alien swarm. I just downloaded it last night and it's excellent for the price (good game for free, what's not to like?) with multiplayer up to 4-5 players. It's basically a fun game. not sure if there are other threads that mentioned this game, but I didn't see it during my quick search in this subforum.
just letting y'all know. have fun!