The Left 4 Dead 2 mod "Back to School" has been in development on and off for over three years, but it's finally launched. If you've not heard of it, it offers a new story and setting in which four completely new heroes strive to survive amid an epidemic. Gameplay-wise, you get six new maps for Versus and Campaign, a new game mode dubbed "Marauder", non-linear design, and more.
See it in action for yourself below, then hit the source at bottom to download. Oh, and if you dig the song, it's "Hold My Hand" by the fantastic UNKLE.
Note these custom weapons and reanimations (seen in the trailer) may be used in conjunction with "Back to School", if you like.