A puzzle-platformer game from the 90's is currently free everywhere for limited time, of course. And that game is Oddworld: Abe's Oddysee from Oddworld Inhabitants?! The devs don't seem to lack sense of humour.
Anyway, at the time of writing the game is free for a little more than 40 hours on Humble Bundle and GOG, while there's no set deadline on Steam yet. The Steam key from Humble Bundle must be redeemed before 10AM PT on October 5. Below are the links to each of the stores:
No matter what method you use, once you successfully add it to your Steam or GOG library it'll be yours to keep forever.