devilutionX, based upon the reverse engineered code from Devilution, has a big 1.0 release for playing the original Diablo on modern systems. Note: You need a copy of the original data to play, which you can get easily from GOG.com.
With the 1.0 release comes some huge improvements like:
- FPS no longer capped at 20hz; smoother mouse and transitions
- Full-featured gamepad support
- All movies can now be skipped via ESC
- Implement --help, --version and --data-dir
- Implement FPS counter (-f)
- Force windowed mode with -x
- Skip startup movies with -n
- Implement scroll wheel navigation
- Implement touch support
- Load assets from the same folder as the mpq
- Faster loading
It's still currently limited to quite a low screen resolution, but the coders responsible for the project did say widescreen support is a goal for it. They're also currently working on supporting the Hellfire expansion. See the release notes here on GitHub.
Love seeing open source revival projects like this, even when it's somewhat of a legal grey area since unlike other open source game engines (openXcom, OpenRA, OpenMW and so on) it was made by reverse engineering the original code. Even so, since it does require the original game files (GOG.com), perhaps that's why Activision Blizzard have left it alone? Or it's so old they just don't care.