Check out the Steam Store here: http://store.steampowered.com/app/261880
It is the integrated expansion pack for "Zombies." which was released last year. The new pack features new levels, new weapons, completely redesigned boss fights, and a pile of new features like game-pad support, civilian NPC's that help you fight instead of following you in a line and getting lost, and the ability to unlock closed doors to get new weapons and powerups.
You can find more information about the game and its predecessor here.
Steam Keys for Desura, thebignic.com, IndieGameStand, Green Man Gaming, and Groupees have been sent. We're still working on IndieGala, IndieBundle, and GamersGate keys - but stay tuned!