The brilliant E3 2006 trailer of Super Smash Bros. Brawl shows Mario performing a screen-filling fireball move that makes Ryu's Hadouken move seem like a pat on the back. How did he do that?
By collecting the Smash Ball, that's how. That awesome fire move is Mario's Final Smash move. Every character has their own Final Smash, which can only be performed after you collect the Smash Ball.
New images of the Smash Ball, and Mario's Final Smash move in action, can be found below.