Which episodes of One Piece are filler?
There seems to be alot less filler in One Piece than in Naruto or Bleach.  That being said, there have been 12 filler arcs:


Warship Island Arc (episodes 54-61)
Post Alabasta Arc (131-135)
Goat Island (136-138)
Ruluka Island (139-143)
G-8 (196-206)
Ocean's Dream (220-224)
Foxy Returns (225-226)
Lovely Land (326-335)
Spa Island (382-384)
Little East Blue (426-429)
Z's Ambition (575-578)
Ceasar Retrieval (626-628)


Episodes 50, 99, 102, 213, 280-283, 291, 292, 303, 317, 318, 336, 406, 407, 492, 499, 506, 542, 590 are also listed as filler on the wiki.

There are additional episodes that are part of main arc that spend time on (sometimes non-canon) details that aren't significant in the manga.  As this is subject to opinion and doesn't take an entire episode, I don't have a simple list for this.