Update: Time's up! Hope you managed to grab it.
Original story: First-person shooter Metro 2033 is free on Steam (opens in new tab) for the next 24 hours, and if you download it before time runs out you can keep it. The spooky post-apocalyptic shooter from 4A Games was released in 2010 and was followed by a sequel, Metro: Last Light. It was also later remastered in Metro 2033 Redux.
But the original Metro is still absolutely worth playing, and it's hard to beat the offer of paying zero dollars for it, especially while you might be waiting impatiently for Metro: Exodus which is due in 2019.
The offer expires at 10 am PT / 6 pm BST / 7 pm CET tomorrow.