Playdead's Limbo, which was originally released in 2011, is free to download from Steam right now.
It's free between June 21-22, so you'll have to act fast. And at that, it's unclear when the free offer will expire, so better to download it sooner rather than later.
Limbo is an atmospheric platformer in which you'll guide a boy across a dangerous world, full of traps and puzzles. It has a Metacritic score of 88 and it's been well reviewed by users since its launch.
The free giveaway is to celebrate Playdead's upcoming title, Inside. There are certainly some parallels to draw between the two, as you can see from the trailers below.