Is ONE PIECE episode 556 filler?
Canon No, episode 556 of ONE PIECE is canon.
#556
Unveiled! The Secret Weapons of the Sunny!
初披露！サニー号の秘密兵器！
Aired 15 Jul 2012
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.