Diablo, the 1996 mouse-destroying hack-and-slash original, is now playable in your browser. The work of Rivsoft (which also specialises in other Blizzard-centric projects, such as this Diablo 3 Character Planner), it gives you access to the shareware build of the game, but if you own the original and have the files on hand, you can access the whole game.
According to the page, the port is based on source code "reconstructed" by two programming teams, which retains all the "bugs and badly written code" from the original game. If you own the game already (it’s available on GOG), you can search for a particular file and drag it onto the browser page to access the full game.
But the shareware version should suffice if you just want a quick go: according to Wikipedia the shareware version features the first two areas of the dungeon, and one of the three character classes.
It’s fun to jump in for 20 minutes or so: I’d forgotten how easy it is just to boot up the first Diablo. There are no overlong cutscenes, no interminable scrolling dialogue sequences, virtually no fanfare at all: you turn up in the town, you walk (very slowly, granted) to the dungeon, and then you start murdering skeletons.
You can check it out here.