This is a list of chess applications that I personally use and recommend.
The first 2 were created by me. You will find tactics generators, tactics trainers, opening builders and more.
Description: Generate chess tactics from your own lichess games.
Description: Answer if it's the best move or a blunder, from your own games. By vitomd
Description: Find mistakes and blunders from your lichess games. By tailuge
Description: Watch your opening tree from your lichess games. By tailuge
You also will find a lot of of chess experiments by tailuge here https://tailuge.github.io/chess-o-tron/
Description: It's like an open source puzzle rush and tactic trainer with many options and modes. By linrock
Description: Investigate each part of Stockfish static evaluation function. By hxim
Description: Great online chess analysis tool, import pgn , evaluate static features (the little s on top). By hximh
Description: Import a pgn file or lichess study and use spaced repetition to learn it. By ArneVogel
Description: Build your repertoire online. By jven
Description: Knight jumps visualization. By hookdump
Description: The free and open source chess server. By ornicar
Hope you find some of them useful. You can check my list of personal projects for more chess projects.