Left 4 Dead Dam It map by Turtle Rock Studios

The best selling and popular "Left 4 Dead" 4player Co-op game was developed by Turtle Rock Studios. Many years have passed and there is no official info from Valve if they want make new L4D3 game also Valve own the IP license to L4D games it's up to Valve if they want make a sequel but they don't do it.
Official Left 4 Dead Blog here is still up and alive.

Turtle Rock Studios made a campaign that was never finished, along with a couple survival maps that never shipped. Dam It stitches Dead Air and Blood Harvest together. Now you can download this map. You can find more info at pcgamer.com or  here.  Notice it has some bugs but still playable and there is some missing textures. "It's gray box, and some code features are missing, but it's playable from beginning to end," he wrote. "It also includes two standalone survival maps that never shipped."

Download: Install info: unzip .vpk file and copy to Left 4 Dead 'addons' folder. It will show up under Menu Extra addons.
Dam It map for L4D1 is 144MB
http://www.gamemaps.com/details/17529
Dam It map for L4D2 port:
http://www.gamemaps.com/details/17533

There was many map mods you could download for L4D games in the good old days at this mod websites but they are closed:
www.l4dmaps.com  Closed have moved to gamemaps.com
www.l4dmods.com  Closed

One last thing a new car simulator game "City Car Driving" have been released at steam it have got some good reviews see it here