Amp

Amps (AKA Electro-Chomps are small electrical metal balls appearing in a variety of different Mario games.

Super Mario 64 & Super Mario 64 DS
They appear in these games and act as a roll similar to Li'l Sparkys and Hotheads. They move around different objects.

New Super Mario Bros.
They reappear as uncommon enemies in this game and can only be defeated by a Mega Mushroom, Blue Shell and Star.

Super Mario Galaxy Series
Amps can be found in many galaxies. They don't have eyes.

New Super Mario Bros. Wii
They once again appear as enemies to Mario, Luigi, Blue Toad and Yellow Toad.

Mario Party Series
They appear in the Mario Party series as obstacles and sometimes items.