Metro 2033 was a diamond in the rough upon its release back in 2010. Set in the subway beneath a post-apocalyptic Moscow, it was atmospheric, bleak, and buggy. Now it's free to keep, if you grab it before 10am PT/6pm GMT on March 15th.
Developers 4A Games have made it available as part of the series' 10-year anniversary, alongside deep discounts on the rest of the series. Last year's Metro Exodus, the third game in the series, is 66% off at £11.89.
Note that what's being offered for free here is the original 2010 release of the game, which while patched, is not as polished as the Metro 2033 Redux version released in 2014. That version added features from the game's sequel, Metro Last Light, including improved AI. Metro 2033 Redux was also offered for free via the Epic Games Store in December, whereas right now it adn Last Light Redux will cost you £3 each.
£3 isn't a lot of money for a better version of the game, but hey, free is still free. If you need more information of exactly what pros and cons you might be in for from the original, Alec's review from back when is a fair summary:
I like Metro 2033, but I wish I loved it. I can't help but want it to be a curious cause to get behind, as with Stalker. It’s one of very few post-Half-Life 2 games that really understands and acts on why people dug that so much, and despite the infuriating checkpoint thing, it’s just obtuse and pleasingly fiddly enough that it feels like a PC game first and foremost.
You can grab Metro 2033 from Steam now.