Since the reveal of amiibo and their cross-game functionality, I'm sure more than a few fans have been wondering if they would be supported by third-party titles in addition to Nintendo's own. The first such title to be announced was Ace Combat: Assault Horizon Legacy Plus, an update to 2011's Assault Horizon Legacy, in which scanning amiibo will give your jet a character-themed paint job. But that won't be available in Japan until next year, whereas another amiibo-compatible game is out right now.
The latest One Piece game on 3DS, entitled One Piece: Super Grand Battle! X, was built with amiibo in mind. According to the above magazine scan provided by the Arlong Park Forums, by scanning the respective figures, you'll be able to dress Luffy as Mario or Luigi, Zoro as Link, and Sanji as Fox. No word yet on whether the rest of the Straw Hat crew will get amiibo costumes too.
Both One Piece and Ace Combat are published by Bandai Namco, which developed the newest Super Smash Bros., hence likely why it is the first third party that gets to incorporate amiibo supprt into its software. As these are 3DS games, you'll of course need the New 3DS to scan amiibo, which is currently not an issue with us in the West since we have neither the hardware nor the games over here yet. Do remember that regular 3DS models will eventually get an amiibo-scanning attachment.