Fairy-Stockfish can be used on various websites online, either as the official playing and analysis engine of the site, or as a bot interacting with it. If you want to experiment with custom variants, Fairyground might be a good place to start.

Fairyground (Fairy-Stockfish playground) serves as a playground to try out Fairy-Stockfish online with custom variants, enabling game play and interactive analysis.

Lichess uses Fairy-Stockfish for play against the computer in variants and for server-side analysis. When it is online you can also play against the official Fairy-Stockfish bot on lichess https://lichess.org/@/Fairy-Stockfish, which uses the latest NNUE networks for variants. You can also set up your own lichess bot with Fairy-Stockfish if you like.

On Pychess you can play various chess variants against Fairy-Stockfish, e.g., Xiangqi, Shogi, Makruk, Janggi, and more.

Lishogi uses Fairy-Stockfish for play against the computer from custom positions and for local analysis. Besides that, there is also an official Fairy-Stockfish bot on lishogi https://lishogi.org/@/Fairy-Stockfish, which uses NNUE evaluation.

Playstrategy uses Fairy-Stockfish for game play and server-side analysis in various games.

Xichess is a Xiangqi website using Fairy-Stockfish for game play and analysis.

To play bughouse against Fairy-Stockfish on FICS, you need to set up Fairy-Stockfish with XBoard/Winboard zippy mode.