Chessstack

Chessstack is a chess repertoire builder and opening trainer. Instead of buying a course, you build your own repertoire move by move on an interactive board, then drill it until you know it.

While building, each position shows moves from master games (players rated 2500+), how often each move is played at different Lichess rating levels, what well-known players chose, and a Stockfish evaluation. You can add notes to moves, import PGN files with variations, and export your repertoire as PGN at any time.

Drills use the FSRS spaced repetition algorithm, the same kind of scheduling Anki uses, so lines you know come up less often and lines you miss come back sooner. You can set your target retention and maximum interval.

Other features:

• Opening trainer: play your lines against a computer that picks replies based on how often real players choose them.
• Opponent prep: pull any Lichess or Chess.com player's games, see what they play, and fill the gaps in your repertoire.
• Game review: import your own games from Lichess or Chess.com and see where you or your opponent left your preparation.
• Puzzles from the openings you actually play.

The free plan includes one repertoire. The paid plan ($1/month or $10/year) adds unlimited repertoires. Chessstack is open source under the AGPL, and the self-hosted version runs in Docker with no limits and no account needed.