Law of the Board (BYOND)

Uses the BYOND client, download from https://www.byond.com/download/ which is completely free. Game is completely free to play as well.

Law of the Board is a chess game which allows editing of board and piece placement, and some rules. This allows most of the standardised variants based on the traditional Western International version, and some historical ones.

Originally requiring games to be hosted to be played multiplayer, the game now plays in local co-op too.