WordNet (r) 3.0 (2006):

naysayer
    n 1: someone with an aggressively negative attitude

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "naysayer":
   anomaly, bitter-ender, brawler, crosspatch, defeatist, defendant,
   denier, diehard, disputant, dissenter, dissentient, dissident,
   freak, individualist, inner-directed person, intransigent,
   irreconcilable, last-ditcher, litigant, misfit, negativist,
   nonconformist, noncooperator, objector, obstructionist,
   obstructive, oddball, opposer, oppositionist, pessimist, plaintiff,
   prophet of doom, protester, quarreler, refuser, resister, sceptic,
   scrapper, sport, wrangler