Crazyhouse Chess.
Basically the same as chess, except you literally ‘capture’ the opponent's pieces, which means you can use them as your own after placing them on any empty square on the board you want. Obviously, you can't place pawns on the last rank. In effect, this makes sacrifices twice as harmful. In vanilla chess, a sacrifice is simply a piece lost. In Crazyhouse, it is equivalent to giving your opponent one of your pieces, so a Knight sacrifice means your opponent gets an extra Knight he can place on board where and when he wills. That means those who have (or think they have) mastered sacrifices in chess will have to think again if their existing strategies give them an advantage any longer or not.
Unrelated note, though, what I absolutely hate about this variant is that you can place the ‘captured' pieces anywhere on the board. If I were designing it, I would probably limit it to the four ranks on the player's own side so that players can't go about capturing a few pawns from the opponent and placing them on the higher ranks (closer to promotion) which seems more like a hack than a strategy when someone plays like that.