The Collaborative International Dictionary of English v.0.48:

muckraker \muck"rak`er\ (m[u^]k"r[=a]k`[~e]r), n.
   A person who habitually muckrakes.
   [PJC]

WordNet (r) 3.0 (2006):

muckraker
    n 1: one who spreads real or alleged scandal about another
         (usually for political advantage) [syn: muckraker,
         mudslinger]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "muckraker":
   Thersites, annotator, backbiter, belittler, calumniator,
   captious critic, carper, caustic critic, caviler, censor, censurer,
   cognoscente, commentator, commenter, connoisseur, critic,
   criticaster, criticizer, critickin, criticule, cynic, debunker,
   decrier, defamer, deflater, depreciator, derogator, detractor,
   disparager, editor, faultfinder, hatchet man, knocker, libeler,
   literary critic, man of letters, mudslinger, railer, reviewer,
   scholiast, slanderer, smellfungus, social critic, textual critic,
   traducer