As promised, the Frictional guys have released the source code for Penumbra: Overture! You can check out their Git repository here, and read their blog post for more details. The source code is free, but the assets are not: to actually run the game you will still have to buy it, either directly from them or from the Humble Bundle (with less than a day remaining!)
Penumbra has a lot of cool tech that's worth checking out, such as the physics interaction, foley sound effects and universal stencil shadows. Here's a tech demo video for Penumbra that shows off some of its features:
I don't know what to expect, but I'm really excited to see what you all do with these newly open-sourced games!