Currently, dystopian underground adventure Metro 2033 can be grabbed on Steam for the grand total of no money. All you have to do is boot up Steam and register the game to your account, and then it's yours to keep.
Released back in 2010, the first-person title was based on the Russian novel by Dmitry Glukhovsky, and tells the tale of post-nuclear war Moscow, where what little humanity remains must live within the city's Metro lines, scavenging for supplies and avoiding hordes of dangerous mutants.
This Steam offer was available for 24 hours, but time is almost up. The offer is expected to close within the next few hours, so snap it up fast. The latest entry in the series, Metro: Exodus, is currently in development for PS4, PC and Xbox One, and is expected to launch February 22.