Valve has released an Expanded Mutation System for Left 4 Dead 2 which is now in beta, according to the Left 4 Dead Blog.
The system, or EMS, was put into place to allow mod authors deeper customization and the ability to create more unique versions of maps in the game.
This is an expanded version of the original Mutation system, which allowed authors to make small tweaks to existing game rules. The new version will use the Squirrel scripting language to expand player options.
So while the old version gave users the ability to customize existing scripts, EMS allows for a more personal experience. While you cannot create new characters, you have more control over existing ones.
The Wikipedia article for the new system gives a few examples for use, including but not limited to spawning and controling entities, and attaching scripts to gameplay events.
Valve has also released a new map called Holdout which will show users how to utilize the new system.
The blog post does not state how long the beta will be up for, but it will eventually be rolled out into the main game.
