Trakt for iOS
Works on iPhone and iPad.
While this season isn't necessary for the main story, I felt that it's pretty fair to slow down and have some random adventures after the craziness of the Alabasta arc, easing the viewer into the next big arc. I've never liked skipping filler personally, and I think this one is worth watching because of certain episodes that have some really sweet character moments; namely, the Usopp episode.