POW Block

The POW Block, also occasionally referred to simply as "POW", is an item that appears in Mario Bros. (the arcade version and its ports; not the Game & Watch one). When hit, it damages all enemies in the screen, but only those that were on the ground. It has the same function in Super Mario Bros. 2, but it can only be used once as the player has to throw it onto the ground to make a POW.

A giant version of the POW block shows up in the Game Boy Advance port of Super Mario Bros 2, as well. Giant POW Blocks can bounce around the floor, causing multiple tremors, before it disappears.

Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island, if Yoshi hits a POW block, all nearby enemies turn into stars. POW Blocks were most commonly won in Bonus Challenges, where they could be used anywhere afterwards, being called Anywhere POWs.

Paper Mario
In the Paper Mario series, the POW Block is an item that does two damage to all enemies (but much more in Super Paper Mario if the remote had been shaken with each corresponding prompt). It penetrates defense and makes enemies with shells (such as Koopa Troopas) turn over. It does not affect flying enemies, however, unless they are on the ceiling.

Super Smash Bros. Brawl
The POW Block's makes an appearance on the Super Smash Bros. Brawl stage Mario Bros. where it can be used to paralyze all enemies on the screen, allowing them to be picked up and thrown at other Players.

Mario Kart Wii
The POW Block is a new item in Mario Kart Wii. When dispatched by a player, all racers ahead of the user will find POW Blocks above their karts. They must jump three times in good timing to destory the POW Block or they will spin out in a similar way to that caused by a Banana Peel. Racers won't be affected by the POW Block if the third strike goes off while they are in mid-air. The player can also avoid it by starting a wheelie right when the third strike goes off, by doing this, they will still spin out and lose items, but they will still keep their speed. They are different in this game, with the biggest difference being blue.

Mario Super Sluggers
POW Blocks also appear in Mario Super Sluggers as a item. They are refered to as a POW-Ball. The item is an unlockable item that will stun the players that are on the field. ''

Trivia

 * In Mario Kart Wii, "POW" can be seen in a license plate of cars in Moonview Highway.