Just yesterday, valve announced that they would be releasing Alien Swarm -- a retail remake of a popular Unreal 2004 mod, now on the source engine -- on steam in three days. Free of charge. It also includes the source code, so you don't need to have Half-Life 2 installed to run it. All it requires is the installation of steam. This also means that like other source-engine games, it'll be fully moddable.
http://store.steampowered.com/app/630/
Looks like more co-op goodness, but damn it Valve, you're making your competitors look bad.
http://store.steampowered.com/app/630/
Looks like more co-op goodness, but damn it Valve, you're making your competitors look bad.