Stockfish

Stockfish is a free, open source chess-playing program that used by top players to improve their game. It is made available under GPL, which requires making the source code fully available in any distributions.

Stockfish 12, released on September 2, 2020, is considered substantially stronger than its predecessors. It is the top chess-playing program in the world.

When the #2 player in the world (Fabiano Caruana) trains with it, Stockfish anticipates games of 35 or more moves in duration.