Continuing their on-going support for Left4Dead 2, a game which I should remind you came out in 2009, Valve announced that they would be adding Steam Workshop support for Left4Dead 2.

Not only will the workshop support custom maps, weapons, and items, but the team also announced that they would be releasing new scripting tools that would allow players to modify the way the game behaves and form their own custom scenarios. These new tools will also allow the creation of new Mutation game modes that can then be used on existing maps or included in a custom campaign. These Mutations will also be made available through the Workshop as well for easy access.

Left4Dead 2 generated some ill will when it released back in 2009 due to its short development time, but moves like this are a great example of how Valve continues to generously reward and support the community and players of their games.