Many years ago a modding group created Alien Swarm on the UT2k4 engine, this was an awesome mod! great at lans! then they started development on a source version. Valve then took them on and gave us gamers a lovely FREE! treat

http://store.steampowered.com/app/630/

Amazing fun online with 3 friends, and its free
Currently there is only one campaign but community created campaigns will be coming out soon I should imagine and then can probably expect some new campaigns from valve themself.