Fairy-Stockfish is used on many chess variant websites, most notably on lichess.org and pychess.org. If you want to experiment with custom variants, Fairyground might be a good place to start.
Fairyground
Fairy-Stockfish playground
Fairyground serves as a playground to try out Fairy-Stockfish online with custom variants, enabling game play and interactive analysis.
Lichess
Chess server
Lichess uses Fairy-Stockfish for game play and server-side analysis.
Pychess
Chess variant server
Pychess uses Fairy-Stockfish for game play, server-side analysis, as well as local analysis.
Lishogi
Lishogi uses Fairy-Stockfish for game play and server-side analysis, mainly for Minishogi.
Playstrategy
Playstrategy uses Fairy-Stockfish for game play and server-side analysis in various games.