It has been 23 years since the original Diablo game released to Windows PCs, followed by launches on the Mac and PlayStation console a couple of years later. Fans of the franchise are hoping that Blizzard will announce a Diablo 4 (as opposed to Diablo Immortal for mobile devices). In the meantime, if you're itching for old school hack-and-slash action, the first Diablo game is playable in a web browser, for free (sort of).
So here is the deal—Rivsoft uploaded a port of the original Diablo game to the web. According to the description, it is "based on source code reconstructed by GalaXyHaxZ and the divilution team." To play the full game, you need to drag a file into the browser (DIABDAT.MPQ). If you don't already own it, you can grab it from GoG for $9.99, or punch the Play Shareware button instead.
The shareware version is free to play, though it does not provide access to the entire game—you get to trod around the first two areas of the dungeons with one of the three playable classes (the other two are locked). Many of the NPCs are not in the shareware version as well.
There is another caveat. As news spread, the site is presumably getting hammered. I have to yet to get the game to load at the original link. In my repeated attempts, I've seen a "network error" message, a "timeout of 0ms exceeded" message, and an attempt to load the shareware version that ultimately stalls out before it finishes.
Other sites have reported success getting the game to load, however, with a bit of persistence. Note that the shareware version is a 50MB download. That should not take very long a decent broadband connection, but it is something to bear in mind if you are working with a tight data cap.
If you are having trouble as well, try this mirror link. I got it to load on my first try. It plays fairly well, though I only spent a few minutes checking it out. Apparently the port left in the bugs that shipped with the original game over two decades ago, but hey, it is playable.