Steam Workshop is coming to Left 4 Dead 2 for better community interaction. Linux support is slated to come the middle of next month.
Steam Workshop for Left 4 Dead 2 is going to allow browsing/rating/discussing community-made add-ons and to install such extras in a very easy manner.
The Left 4 Dead blog announcement
goes on to mention, "All of this will also be available not just on the PC and Mac, but also for Linux users as well starting in the middle of next month and rolling out from there."
More details coming.