WordNet (r) 3.0 (2006):

nitpicker
    n 1: someone who makes small and unjustified criticisms

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "nitpicker":
   belittler, captious critic, carper, caviler, censor, censurer,
   common scold, critic, criticizer, equivocator, faultfinder,
   frondeur, hairsplitter, hedger, momus, mystificator, mystifier,
   obscurantist, palterer, perfectionist, pettifogger,
   picayune critic, precisian, precisianist, prevaricator, pussyfoot,
   pussyfooter, quibbler, scold, shuffler, smellfungus, stickler,
   tergiversator