The third entry in the Metro series, Exodus, is still months away from launch with its new February 2019 release schedule, but the original Metro 2033 that started it all is being given away for free on Steam today.
The giveaway is for the classic version, not the Redux edition that updated the game to Metro Last Light's engine. However, the experience will still be worthwhile for anyone who hasn't given the very atmospheric series a go yet.
Here's a short description of Metro 2033's setting from the Steam store page:
In 2013 the world was devastated by an apocalyptic event, annihilating almost all mankind and turning the earth’s surface into a poisonous wasteland. A handful of survivors took refuge in the depths of the Moscow underground, and human civilization entered a new Dark Age.
The year is 2033. An entire generation has been born and raised underground, and their besieged Metro Station-Cities struggle for survival, with each other, and the mutant horrors that await outside.
Note that the promotion comes thanks to a Deep Silver publisher sale, so the rest of the Metro series, as well as other games from the company, are heavily discounted for the weekend.
Metro 2033's giveaway can be found here, with it lasting for 24 hours. The shooter will be permanently added to the Steam library of anyone who claims it during that time period.