Seriously people - if you don't have L4D2 - get it NOW!!!
NOW NOW NOW NOW NOW!!!!
One of the best team based games since Counter Strike imho...
8 Story Campaigns - multiple multiple multiple competitive versus modes - new free updates all the time - and built from the ground up MOD friendly!
I've already put nearly 50 Hours into it and I've yet to try nearly 40% of the available content.
The combo L4D 1&2 pack is also a good recommend - because there are already amazing MOD's for the first game - but honestly some of the first is already being ported and rebalanced for the L4D2 engine by the Devs - and there is reason to believe this will continue.
Seriously tho for 7 or 10 bucks - you won;t go wrong - this is an INSANE value!!!