The Collaborative International Dictionary of English v.0.48:

Detractor \De*tract"or\, n. [L.: cf. F. d['e]tracteur.]
   One who detracts; a derogator; a defamer.
   [1913 Webster]

         His detractors were noisy and scurrilous. --Macaulay.

   Syn: Slanderer; calumniator; defamer; vilifier.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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