Valve recently opened up Left 4 Dead 2′s Mutations for customization by the community. Mutations are special modes that alter the rules of the game. Back when I still played it, the mutations could be as simple as constantly draining health for the human players or something a little more terrifying, like constant waves of Tanks. They’ve undoubtedly been used in more interesting ways since then, and soon, we can look forward to even more interesting Mutations.
The EMS is a “vastly improved scripting system” that gives modders better tools with which to work their modding magic to “create deep custom experiences that can be played on any map or tailor create something for new or existing maps.” No word yet on when it will make its way to the game, but I predict something along the lines of it’ll be “done when it’s done.”