The original Diablo released in 1996 is now playable right on your computer's web browser.
The browser port was developed by Rivsoft, a team of developers who work on several Blizzard-related projects. However, the port wouldn't be possible were it not for the work of a development team who in 2018 reconstructed the original soucre code for Diablo.
The Devilution team and GalaXyHaXz reverse-engineered the source code for the original Diablo last 2018. The project was meant to be archival, so the source code was reconstructed with all the bad code and bugs from the original 1996 game.
From there, the source code was used to produce the browser port. Check out the Diablo game here. It's not the full game though.
It is a shareware version of Diablo which limits players to only one of the three character classes, and the first two areas of the dungeon.
If you purchased the original game on GoG though, you can drop the original DIABDAT.MPQ file into the browser page and start playing the full game. Anyone looking to play the full version of Diablo for free will have to look elsewhere.
Still, if you have nothing to do at the moment, this is a really easy way to get a quick version of Diablo up and running on your computer with minimal fuss.
Meanwhile, Blizzard is hard at work on Diablo 4, as well as a mobile title called Diablo Immortal.