There are a lot of chess variants, in which there are extended complete sets of chessmen. I would like to offer one more with small variations.
The game is conducted by rules of International Chess with the exception of the board and setup, and the Castling rules.
Castling has most of the same restrictions as in orthodox chess: neither the King nor the Rook may have previously moved, the King may neither be in check nor move through check, and all of the squares between the King and the Rook castled with must be empty. However, when castling in Big Chess, there are more choices of where to castle to. The King may castle one, three or five squares towards the Kingside Rook, or two, four or six squares toward the Queenside Rook. In all cases, the Rook moves to the square on the King's other side.
There is an implementation of Big Chess for Zillions of games. You can download it here: