Crazy Hand is a secret boss in Super Smash Bros. Melee and Super Smash Bros. Brawl. It can be fought during Event Match 50 or in Classic mode if the difficulty is at least set to Normal and less than 18 minutes have passed by the time the Master Hand is faced and has only half HP. He is unlockable as a Trophy too by beating Adventure mode on at least Hard difficulty with no continues.
In Brawl, he's a boss in the Subspace Emissary controlled by taboo.