Game Courier supports many different games. Here is one picked at random:
Mini Thunder Chess - A small-scale hybrid of Metamorph Chess, Fusion Chess, and Assimilation Chess.

Intrigue Chess

If you had a Java-capable browser, you could play Intrigue Chess here.
Keys "s"=save "l"=load "b"=back once

A secret pawn carries the location of the king.

I programmed this so that pawns promote to queens, and did not allow castling. If a king appears on the square already occupied by the other, it is an automatic win.

Intrigue Chess by Nigel Chapman.

Chess Variants