The Collaborative International Dictionary of English v.0.48:

disparager \dis*par"a*ger\, n.
   One who disparages or dishonors; one who vilifies or
   disgraces.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disparager
    n 1: one who disparages or belittles the worth of something
         [syn: detractor, disparager, depreciator, knocker]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "disparager":
   Thersites, backbiter, belittler, calumniator, caustic critic,
   cynic, debunker, decrier, defamer, deflater, depreciator,
   derogator, detractor, hatchet man, knocker, libeler, muckraker,
   mudslinger, railer, slanderer, social critic, traducer