ChessAtlas

ChessAtlas is built around a single conviction: an opening repertoire is only worth as much as the part of it you can recall under clock pressure. Most trainers stop at letting you build lines; ChessAtlas treats the repertoire as something to be maintained, scheduling reviews with the FSRS algorithm - a more efficient successor to the SM-2 scheduling that older spaced-repetition trainers use - so study time concentrates on the moves most likely to be forgotten.

The feature that sets it apart is deviation detection. Link a Lichess or Chess.com account and ChessAtlas imports real games, then flags the exact move where play left the prepared line. Instead of reviewing a repertoire in the abstract, a player sees which variations are actually breaking over the board and can feed those positions straight back into the review queue. Courses can be built from scratch or forked from a public library, which lowers the cost of starting.

The trade-offs are scope and maturity. ChessAtlas is web-only, with no native mobile app yet, and it does one thing - opening repertoire training - so it is not a play platform, puzzle trainer, or engine. The course catalog is younger and smaller than Chessable's. For an intermediate-to-advanced player whose results hinge on holding prep, that focus is the point; a beginner who needs to learn the game broadly is better served by a full platform first. (Disclosure: ChessAtlas is built by ChessDir's editor; it is reviewed here on the same criteria as every other app.)