ChessX is an Open Source chess database - new version 1.1.0

Scid - Chess Database Software

Chess Openings Chess Opening Software

learning - Where can I find free training tools like detailed annotated chess openings, middle games and endgames in the PGN format? - Chess Stack Exchange

ChessX Flathub

Small Encyclopedia of Chess Openings ABCDE on CD (3rd Edition)

We made a tool that lets you search the lichess game database by chess concepts. Link in the comments : r/chess

The Outpost Chess and Opening Master Partner to Create Global Chess Community for Players, Coaches, Arbiters, and Clubs

Databases: Tips, Tricks, and Softwares

Introducing the June 2023 Release of Opening Master Chess Database: Get Ready for Summer Tournaments!

How to Use Chess Engine and Databases Like a Pro - TheChessWorld