Valve don't force people to use Steamworks, however it will not work with another companies system. That is not anti-competitive.
I'll be the driver
The game is called STALKER: Call Of Pripyat
You could argue that L4D2 is what L4D1 should have been ... BUT ... L4D2 is, thankfully, fantastic, the new special infected and better level...
Separate names with a comma.