There's a free game on Steam by Valve known as Alien Swarm. Free is a pretty good price last I checked, so you should partake in this updated-to-Source UT mod.

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

If you're getting "Steam Servers are too busy" change your download location to another Steam content server by going to View - Settings - Downloads + Cloud - Download Region and simply cycle through download regions until one frees up.

Happy <verb>ing!