Zero Suit Samus

Zero Suit Samus is, simply, Samus Aran without her power suit. She first appeared in this form as a playable character in Metroid Zero Mission. In Super Smash Bros. Brawl, Zero Suit Samus is a playable character. She can be played once Samus performs her final smash, Zero Laser, which will overheat her regular power suit.

While losing power, she gains speed. In Super Smash Bros. Brawl, she has the ability to use her gun as a plasma whip, and to paralyze opponents.

Metroid series
Samus in her Zero Suit has appeared in Metroid: Zero Mission, Metroid Prime Hunters, Metroid Prime 2: Echoes, Metroid Prime 3: Corruption, and Metroid: Other M (as well as Metroid Prime Trilogy and Super Smash Bros. Brawl, seen below). Zero Suit Samus is usually not playable in the games she's appeared in excluding Metroid: Zero Mission and Metroid: Other M.

Super Smash Bros. series
Zero Suit Samus was among the earliest newcomers announced for Super Smash Bros. Brawl. She appears after Samus Aran uses her Final Smash, called Zero Laser. Zero Suit Samus has a completely different moveset than Samus Aran, making her a unique character (similar to Princess Zelda's Sheik form). In Brawl, she is voiced by Alesia Glidewell. Note that if Samus performs a Final Smash while wearing the Zero Suit she will revert back to her Varia suit after the Final Smash is over.

Fatal Frame IV: Mask of the Lunar Eclipse
In the Wii video game Fatal Frame IV: Mask of the Lunar Eclipse, which was published by Nintendo, the character Ruka Minazuki can wear two Nintendo themed costumes, one of Luigi (because of Luigi's Mansion, which like this game is centered around the super natural) and Zero Suit Samus.