The Frictional Games Store is running a Halloween sale for one week. What that means is Amnesia and Penumbra are on sale for 75% off.
The games will NOT activate on Steam. They are DRM-free. Demos are available for both.
Amnesia: The Dark Descent = $5.00
Penumbra: Collection = $5.0
The soundtracks for the games are also on sale for 50% off.
Penumbra Soundtrack = $5.99
Amnesia Soundtrack = $5.99
The deal ends on November 3, 2012.