Gog has added a couple of games to their "always free" set:
-
Lords of Midnight -
Doomdark's RevengeI know nothing about these games (never heard of them before) other than they're from the mid-1980's. But if you're into retro gaming, the only thing you're risking is your time. I think GOG has been selling those games for about 5 years (but I'm not sure about that).
You can find all of GOG's permanently free games on this list:
https://www.gog.com/games?price=freeHowever, that list also includes free DLCs and demos which take up about 75% of the list.