Blizzard - Diablo II
The Arreat Summit

GLIDE3-to-OpenGL-Wrapper specifically written for Diablo 2.
System Tweeks - Less Lag

PlugY, The Survival Kit by Yohann Nicolas.

The Moving Caravan A Diablo II Time Traveling Community (Original, Not Resurrected).
The Phrozen Keep Serving D2 Mod Makers Since 2000.

Battle.net Support - Patching Classic Games

GitHub - fearedbliss/Cactus A Modern Diablo II Version Switcher & Character Isolator (Contains: Singling).
GitHub - ChaosMarc/D2VersionChanger Diablo 2 Version Changer (Classic 1.00 - LoD 1.14d).

OpenDiablo2:

• GitHub - OpenDiablo2 repositories
• GitHub - OpenDiablo2/AbyssEngine A 2D ARPG game engine. And GitHub - Ziemas/AbyssEngine with some more updates (but no commits in 2022).
• GitHub - OpenDiablo2/HellSpawner The OpenDiablo2 toolset.
• GitHub - OpenDiablo2/OpenDiablo2 archived.