Stockfish

Stockfish is the strongest open-source chess engine in the world. It consistently tops computer chess rating lists and has won the Top Chess Engine Championship (TCEC) and Computer Chess Championship (CCC) multiple times. Stockfish is completely free, released under the GNU General Public License, and anyone can download, use, or contribute to its development.

The engine powers analysis on virtually every major chess platform. Lichess uses Stockfish for its built-in analysis board. Chess.com integrates it into game review features. Thousands of independent chess apps and tools rely on Stockfish as their analysis backbone. In 2020, Stockfish incorporated NNUE (Efficiently Updatable Neural Network) technology, dramatically increasing its playing strength.

Stockfish is maintained by a global community of developers who contribute improvements through a rigorous testing framework called Fishtest. The project is a model of open-source collaboration, with hundreds of contributors and a transparent development process hosted on GitHub.