The Metal Box, known in Japan as Green Block (緑ブロック, Midori burokku), is an item from the Mario series that appears in Super Smash Bros. Melee and Super Smash Bros. Brawl. The Metal Box encases the character's bodies with metal for a short period of time. Images of each character's metal form can be found here.
The Metal Box encases the character's body in metal. In both games, it has the following effects:
The effect of the Metal Box is not quite as drastic as the various Metal fights in Classic Mode, which last the whole battle and does not wear off. Additionally, the character will still lack voices of any sort when KO'd.
The Metal Box does not appear in SSB, but Metal Mario appears as a foe the player must battle in the 1-Player Game. Metal Mario here can take well over 300% damage before being sent flying off-screen.
 Super Smash Bros. Melee
The Metal Box makes its debut in Melee. In this game, when characters break the Metal Box with a physical attack (or a direct attack such as a sword), their whole body will be encased with metal. The metal condition lasts around twelve seconds and then fades away. During this period they are approximately 3 times heavier than normal. If a projectile destroys the Metal Box, then no one can receive the metal condition. Interestingly, Ness' dash attack destroys the box, but does not give him the metal status. His yo-yo does, however. Characters can fight Metal Mario and/or Metal Luigi in Adventure Mode. In addition, they also fight a random metal character in Classic Mode. The Metal Box also appears as a collectible trophy.
Depending on the weight and walking style of a character, when characters walk, they make a distinctive sound.
 Super Smash Bros. Brawl
The Metal Box appears again in Brawl with its effect unaltered, although it will not react to projectiles. In addition, a "Metal" option exists in Special Brawl, which gives every player a permanent Metal Box effect. It also has an effect on the new ability to swim; while characters will not sink as Metal Mario did in Super Mario 64, they will submerge deeper when falling into the water, which is used to reach a secret area in the Glacial Peak level of the Subspace Emissary mode (3 metal boxes are provided nearby). In Classic Mode, the player must also fight a random metal character. Additionally, the Metal Box also appears as a collectible trophy. The walking sound effect in Brawl is not the same as in Melee; everyone has the same sound when walking.
 Trophy Description (Melee)
First spotted in Super Mario 64, the Metal Box holds the power to turn your character briefly into heavy metal. Your mass is exponentially greater while metallic, making it difficult for others to throw you. Be careful, though: not only will you move a bit slower, but you'll also fall much more quickly than you ordinarily would.
 Trophy description (Brawl)
A green box that transforms whoever attacks it into metal. In metal form, you're a lot heavier than normal, so middling and even strong attacks don't move you too much, and standard attacks won't even make you flinch. The time you spend in metal form decreases with the damage you take. In Super Mario 64, the weight of the metal form allowed you to walk underwater.
Metal Boxes come from Super Mario 64, wherein Mario could destroy them (only after activating them by pressing the Green "!" Switch) so that they release a Metal Cap. Once Mario puts this cap on, he himself becomes metal. He becomes powerful, able to walk underwater, and immune to all damage, but is much heavier and more sluggish that he can't swim. These effects are similar in Melee and Brawl.
Interestingly, Metal Boxes were never referred to by that name in Mario 64; they were simply referred to as "green blocks". Also, although they completely remove characters' voices in the Smash Bros. series, Metal Mario only loses his voice while underwater.