In-game DLC feature announced for Steam
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Publishers will now be able to deliver in-game DLC via Steam, Valve's announced.
The first game to take advantage of this new capability is The Maw, by Twisted Pixel Games.
The extra content's levels titled The Maw: Brute Force and The Maw: River Redirect which fit between the original levels as "deleted scenes."
Any game can use the feature, whether it was originally intended for distribution via Steam or not.
More through there.