May or may not be useful, but I just read that the Alien Swarm source will be released next Monday. Never played it, but wasn't Alien swarm an Unreal tournament mod?
Original post
On Monday, July 19th Alien Swarm is going to be released for free via Steam.
In addition to the game, Valve will also release the complete code base for Alien Swarm. This includes updates to the Source engine SDK and full Steamworks integration. If you’ve ever thought about developing a mod on the Source engine with Steamworks, this release provides more insight and examples for using Steamworks in game production.
Perhaps it's not the engine itself, don't know, nor am I inclined to dig deeper. But maybe this news is useful to someone...