Alien Swarm MapPack 1.2 is a collection of custom maps, campaigns and enhancements for Valve's Alien Swarm, a popular 3rd-person co-op shooter video game, based on Source engine.

The MapPack adds new content to the game, hugely boosting the game's replay value, while not changing its natural gameplay. Extensive testing has been performed to ensure no compatibility issue between various content exists, and the MapPack is both server and client compliant, including popular server mods. Other maps and mods may or may not be compatible with it.

It is based on publicly accessible 3rd party's authored work, with respect to original author's rights and requests, currently featuring no modification to their work.

The MapPack will be maintained and updated for unforeseeable future, adding even more fun for all Alien Swarm fans over the entire world, so please stay tuned and report any issues and suggestions.

The Alien Swarm community will be thankful to have you include this MapPack on your Alien Swarm server! Also, please promote Alien Swarm to your co-op teammates whenever possible!

see: 
http://forums.steampowered.com/forums/showthread.php?t=2642967 
http://www.moddb.com/games/alien-swarm/addons/alien-swarm-mappack-12