Perhaps ironically for a game that started out on the Unreal Engine, Alien Swarm has turned out to be Valve's response to the Unreal Development Kit. It isn't quite as tidy a package as UDK - as a Steam account and 2GB download are required before mods can be played - but it's pretty close, and Steamworks fills in the indie licensing side of the equation. Like UDK however, there is a limited supply of content: Source's standard HL2 materials, models and sounds are not available.
In related news, Swarm documentation has gone up on the Valve Developer Community. Included is a guide to mapping (with the new TileGen as well as in Hammer), and a C++ code overview.
Oh yeah: the game is out too.