Is ONE PIECE episode 339 filler?
Canon No, episode 339 of ONE PIECE is canon.
#339
One Unnatural Phenomenon After the Next! Disembarking on Thriller Bark!
怪現象ぞくぞく！スリラーバーク上陸
Aired 20 Jan 2008
Mochibayo compiled this list from MyAnimeList's labels, because MyAnimeList's episode data is not reaching us at the moment.