Frictional Games have their games on sale through Steam today (probably in honour of SOMA coming out in a few days time), and have also made Amnesia: The Dark Descent free.
As usual with this sort of thing, it’s a limited offer. From right now until 10am (Pacific US) tomorrow, The Dark Descent will cost you no Pounds, Dollars, Euros or any other currency to add to your library and install.
The other Frictional Games have various savings. Penumbra: Black Plague Gold Edition (which includes the Requiem expansion) is $2.00 USD, and so is Penumbra Overture. Those two pre-date Amnesia: The Dark Descent.
Amnesia: A Machine For Pigs (made by TheChineseRoom rather than Frictional), is $4.00.