Load of extra community content to download and try out from an Alien Swarm community I used to be a part of.

"AS Map Pack 2010 - Complete".
http://swarmarmory.com/maps?action=view&id=373-AS+Map+Pack+2010+-+Complete
.torrent - 751mb
.rar (Megaupload) - 751mb

"Spring 2011 map pack".
http://swarmarmory.com/forums/viewtopic.php?f=5&t=989&p=6261#p626 (See this forum post for "quality" ranks for each of the maps.)
http://swarmarmory.com/maps?action=view&id=398-Spring+2011+map+pack
.torrent - 708mb.

Hopefully this'll keep people busy in the game :D