I know we've mostly played L4D2, but Alien Swarm is a pretty great game too. And it's free. Here's a big map pack compilation that came out fairly recently:

http://swarmarmory.com/maps?action=view&id=398-Spring+2011+map+pack

Some new campaigns for us to try:
I Hate Mountains - http://www.l4dmaps.com/details.php?file=8500
Dead Before Dawn Director's Cut - http://www.l4dmaps.com/details.php?file=7774
Dead Before Dawn Too - http://www.l4dmaps.com/details.php?file=7221
Tour of Terror - http://www.l4dmaps.com/details.php?file=8387

Ones you might have but should get if you haven't:
One 4 Nine - http://media.steampowered.com/apps/l4d2/maps/l4d2_one4nine.vpk
Two Evil Eyes - http://media.steampowered.com/apps/l4d2/maps/2evileyes.vpk
Haunted Forest - http://media.steampowered.com/apps/l4d2/maps/hauntedforest_v3.vpk
Detour Ahead - http://media.steampowered.com/apps/l4d2/maps/detourahead.vpk
City 17 - http://media.steampowered.com/apps/l4d2/maps/city17l4d2.vpk
Suicide Blitz 2 - http://media.steampowered.com/apps/l4d2/maps/suicideblitz2.vpk

from here: http://media.steampowered.com/apps/l4d2/maps/suicideblitz2.vpk

(so this is how announcements work eh)