You have about 20 hours left to take advantage of GOG's DRM Free sale, put togeether to celebrate today being the longest day of the year. The games are free of the lumpen, ugly additional code that bootstraps their code to your PC. If for some reason you own a million PCs, you would be allowed, nay encouraged, to install them on each and every one. I wouldn't, because that would involve a lot of work, not least in acquiring an entire planet to source the resources for such a PC collection, but the hypothetical scenario still stands. You really should look over the whole list, but below I've gathered a few treats to entice you in.
If you've somehow managed to miss the previous 156,344 times space-based roguelike FTL has been on sale, it's now only $2.50. Worth it for such an adventure-filled space romp. Another top-down treat, the horrendously sick and wonderful Hotline Miami is all the threes, $3.33.
While there are plenty of brains to splat, for something more traditionally cerebral there's the Wadjet Eye selection, which have an 80% discount when bought together (for $8.96), or you can grab any game from the pack for 50% off. If that's not taxing enough, the Turn-Based Strategy Pack for $12.43 includes Sid Meier's Alpha Centauri, Master of Magic, Master of Orion 1+2, Sid Meier's: Colonization, Fragile Allegiance, Galactic Civilizations I: Ultimate Edition, and Space Empires IV: Deluxe.
Oh, and my personal recommendation for something odd, charming, and cheap is Miasamata. It's only $3.74.
Feel free to recommend games in the comments, because I've definitely missed loads.