Excuse the rapidly-written post, but I only just noticed this and, at the time of writing, there are only thirty Earth minutes until the offer expires. 1997 platformer, and first game in the ongoing Oddworld series, Abe's Oddysee is currently free on Steam. But not just in a free weekend sort of way - add it before 6pm UK time / 10am Pacific today and you get to keep it forever.
Abe's Oddysee isn't to be confused with recent semi-remake Oddworld: New'n'Tasty, although both are the same games at heart. I guess the idea is that if you like what you get for no-pennies here, you might think about buying the new one, or the various others in between. They're all currently reduced by 75%, with the exception of New'n'Tasty, which is 50% off. An 'Oddboxx' containing all of them bar New'n'Tasty is a mere £2.50 until the offer expires in just over 24 hours' time.
I can't help you with that decision - I haven't played most of them because I'm not a platformer man, but I know they have their fans.