In one of those “blink and you’ll miss it” opportunities, Frictional Games currently has the Amnesia Collection, which includes Amnesia: The Dark Descent and its sequel, Amnesia: A Machine For Pigs, available on Steam for free. The trick is that there’s a weird thing where the individual games are free, but the regular link to the Amnesia Collection is still full price. You have to head here in order to access the collection in its free form.
Now, this is apparently for a limited time, but there’s been no indication as to when this offer expires. So if you’ve never had the chance to play these games, get on it immediately.