There is this chess variant in India and we call it 'Supply'. Its a 2 vs 2 game with 2 boards and 2 players of a team take alternate colors. The standard rules of chess apply, but with one twist. If your team member captures a piece on his board he can give it to you and since you are your teammate's opponent's color, you can introduce that piece on your board and continue playing, while introducing the piece on your board. There are few rules like no checking and no pawn promotion by putting it on the last rank. The first team to get checkmated loses. This mod of chess is really fun. I don't know if people play this outside India, and what it is called. I would like to play it online with my friends. Do you know any place online where I can play this variant?
Another variant is crazyhouse. Here when you capture a piece, it changes color and becomes one of your drop pieces. Unfortunately this also allow place mate.