Co-op Zombie survival game using the HL2 engine, looks good!
Eugene, Marcus, and other Survivors return in the spiritual successor of the popular Half-Life 2 Modification Zombie Panic: Source! Contagion is a cooperative survival horror FPS built on the latest Source Engine where players are forced to watch their ammo and aim carefully otherwise be overrun by the zombie hordes.
http://store.steampowered.com/app/238430
This is now available as early access on steam for £2.99 special offer