The Collaborative International Dictionary of English v.0.48:

Complainer \Com*plain"er\ (k[o^]m*pl[=a]n"[~e]r), n.
   One who complains or laments; one who finds fault; a
   murmurer. --Beattie.
   [1913 Webster]

         Speechless complainer, I will learn thy thought.
                                                  --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

complainer
    n 1: a person given to excessive complaints and crying and
         whining [syn: whiner, complainer, moaner,
         sniveller, crybaby, bellyacher, grumbler,
         squawker]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "complainer":
   bellyacher, complainant, crab, crank, croaker, faultfinder,
   frondeur, griever, griper, grouch, grouser, growler, grumbler,
   kicker, kvetch, lamenter, malcontent, mourner, murmurer, mutterer,
   querulous person, reactionary, reactionist, rebel, sorehead,
   sourpuss, whiner