If you've been kicking yourself for missing out on a free copy of The Amnesia Collection on Humble Bundle in January, redemption is nigh. Currently Frictional Games' creepy horror bundle is free on Steam for a limited time, you just have to add it to your collection.
To claim both Amnesia: The Dark Descent and A Machine for Pigs simply navigate to either product page on steam (for example) and click on the free package that includes both games. Voilà, both games are now in your Steam library permanently. Normally The Amnesia Collection is £26.99, with each game priced individually at £14.99.
Amnesia: The Dark Descent and A Machine for Pigs are both cross-platform titles, meaning that you can buy them once and play on PC, MacOS and SteamOS. The atmospheric nature of The Dark Descent in particular lends itself well to a big screen experience in an otherwise darkened room.
Find out more, including the system requirements, at http://store.steampowered.com/app/57300/Amnesia_The_Dark_Descent/. Currently there is no expiration date for this offer, but there's no reason not to jump on it immediately.