Evilcorp
Evilcorp is a one map campaign for Alien Swarm by GRRRR aka Schleicher. This is the original version but packed in VPK with fixed file conflicts, crashes and download link.
EvilCorp has stolen the Parasite from Mars! Kill Everything!
Download
http://gamebanana.com/maps/192263
http://www.moddb.com/games/alien-swarm/addons/evilcorp
Installation Instructions