Blizzard Entertainment: Legacy Games

The Lurker Lounge - Jarulf's Guide to Diablo and Hellfire. Available in online browsable HTML format here.

GitHub - diasurgical/devilutionX a Diablo build for modern operating systems.
GitHub - diasurgical/devilution Diablo devolved - magic behind the 1996 computer game.

The Lurker Lounge - Diablo Backwards Compatibility Guide
Battle.net Support - Rainbow or Washed Out Colors on Windows 7
Diablo Wiki - Command.txt - Unlocking the Undocumented Features

Khanduras Network Diablo modding site.

Diablo Roguelike is a roguelike “unmake” of the popular Blizzard game Diablo. Originaly created for the 7 Day Roguelike Competition.

Hellfire - The Dark
Diablo: The Hell
Unofficial Hellfire 1.02 Mod

freeablo is a modern cross platform reimplementation of the game engine used in Diablo 1. As it is just an engine, you will need the original data files to play the game. GitHub - wheybags/freeablo

GitHub - dgengin/DGEngine is an implementation of the Diablo 1 game engine.

IPXWrapper is a wrapper library which emulates IPX support while tunneling the packets over UDP. This allows you to run legacy games and other software that require IPX without having to install the IPX driver which is unavailable as of Windows Vista/Server 2008.