Oddworld Abe's Oddysee, the classic platformer from Oddworld Inhabitant is one of the most popular retro games available for PC. And you can get the game for free over the next day and a half.
If you want to play the DRM free version of the game, you can get it from the GOG giveaway, by logging in to your account and visiting the homepage and clicking on the "Get for free" button. Or, if you want the Steam version, open the client on your PC, and visit the game's page, and click on the Install game to add it to your library.
The game is also being given away on Humble Bundle, but it is also a Steam key. If you are wondering what's the point of this if the game is free on Steam already. But let's say you want to send the key to a friend who is away from his PC, (or if you yourself are away from your PC), and can't get the game within the next 36 hours, you can get the key from Humble Bundle and send it to them. That's pretty cool. Do remember that you have to redeem the key by 10AM Pacific on Thursday, October 5th, 2017.
Back To Bed, the indie platformer game from Bedtime Digital Games, is also free on Steam for the next 36 hours. You can get it from the game's page in your Steam client.