According to Valve, DLC can now be added to any game on Steam, regardless of whether it was originally purchased via Steam, at retail, or via other digital outlets. It is also a feature of Steamworks, the suite of free tools and services available to game developers and publishers.
The first game to take advantage of this new in-game DLC capability is The Maw, by Twisted Pixel Games.
"We’re happy that we can now offer Steam customers significant expansions to the Maw story, delivering more Maw directly to gamers while they’re still playing the game," says Michael Wilford,Twisted Pixel CEO.
Their first DLC releases are two levels, dubbed The Maw: Brute Force and The Maw: River Redirect. Each DLC level expands The Maw storyline by fitting in-between the original levels as "deleted scenes."