Up for a claustrophobic tunnel crawl packed full of mutants, marauders and otherworldly anomalies? The Russian underground of Metro 2033 should be your next stop, and it's completely free on Steam today. For the next twenty hours (give or take), just check out its Steam page, click to add it to your account and keep it forever. This is the original version of 4A Games's shooter, rather than the polished up Redux edition, but still worth the trip if you've never tried it before.
Based on a series of novels, Metro tells the story of Artyom, young survivor of the kind of weird, reality-bending apocalypse that only seems to happen to Russia. Having the surface world nuked was bad enough, but the atmosphere is now toxic and strange mutant monsters roam the land. Even the underground is plagued by ghosts and anomalies that feel like they're lifted almost wholesale from the STALKER series. This world is a really rough place, and just top top it all off, there's a thriving faction of Nazis living underground with you. No guilt shooting them, at least.
While not as pretty as the Redux version of the game (currently bundled with its sequel on Steam for under $10), Metro 2033 is still no slouch graphically. If anything, it pushed PCs of the time far too hard, and modern hardware can still chug a bit if you try maxing out its vast number of detail sliders. All told, it's a solid shooter, although due to much of it literally being set underground, progression is quite linear. The sequel Last Light opened things up a bit, and the upcoming Metro Exodus will be set in large outdoors environments.
One small tip: While Metro 2033 does have multiple endings, the 'good' ending is not only extremely hard to achieve, but it's also not canon. Metro: Last Light (which picks up the story moments later) just assumes you got the regular end, so don't sweat trying to be the nicest guy in the underground - it's more fun just to bodge your way through things.