Here's an interesting variant by the inventor of Racing Kings (Vernon Parton). The rules are described here:
http://www.chessvariants.org/winning.dir/neutral-king.html
Basically, neither player has a king of their own, but there is a neutral king that may be moved by either player. The object of the game is to checkmate the Neutral King and prevent your opponent from doing so. I don't think there are ANY websites that have it! (Although there is a java applet at pathguy.com where you can play against a computer. But there's nowhere you can play against real opponents.) Brainking could have a world first!