Lichess is great and they mention FICS in one of their blog posts as a great way to play too: https://lichess.org/blog/WrbfjyYAAMOualZ5/our-favourite-open-sourc

Lichess is great and they mention FICS in one of their blog posts as a great way to play too: https://lichess.org/blog/WrbfjyYAAMOualZ5/our-favourite-open-source-chess-websites https://lichess.org/blog/WrbfjyYAAMOualZ5/our-favourite-open... They also mention kfchess, which I didn't know about, and other open source educational, puzzle, variant chess sites and the lc0 engine based on AlphaZero.