Puzzle-platformer Oddworld: Abe’s Oddysee is being given away for free as part of GOG’s ongoing sale. It’ll be free for the next 44 hours or so (at the time of writing), and, once claimed, will remain in your GOG list permanently.
You’ll need a (free) GOG account in order to claim Oddworld, but that’s about it. Then you can start trying to escape Rupture Farms with your fellow Mudoken prisoners.
The main sale runs until 25 September (at 10pm UTC) and includes more games than it would be sensible to list in one place. GOG’s own announcement post features a few of them, like Rebel Galaxy (75% off), Her Story (75% off), Prison Architect (80% off) and Satellite Reign (also 80% off).