Steam, Tracks, Trouble and Riddles is a mod that gives a very good first impression. After a well presented main menu, a slick comic-book introduction brings us forward to the year 2525, via an alternate history in which the failure to resolve the Cuba Missile Crisis leads to the extinction of humanity and the rise of the machines. It’s an intriguing backstory, and the promise of exploring a retro-futuristic world run by B-movie robots is definitely a compelling one.
"One 4 Nine" is a community created Left 4 Dead campaign which takes place in a zombie infested Nevada Desert setting. Featured by Valve themselves, the five-chapter campaign presents detailed map design and engaging gameplay similar to that of Left 4 Dead's own levels.