For all of you who love co-op gaming, give Sven Co-op a try. It is a free co-operative game built on Valve's Half-Life, just released on Steam. You can play the the original HL campaign with friends or try nearly 1,000 custom maps available at our sister site SCMapDB.

http://store.steampowered.com/app/225840

Valve has recently released the official L4D mapping tutorials. There's 21 step-by-step tutorials covering many aspects of L4D mapping.

Visit http://L4DMapDB.wikidot.com for details,
or download the zip directly at http://www.l4d.com/LevelDesignDocs.zip