The Collaborative International Dictionary of English v.0.48:

Complain \Com*plain"\ (k[o^]m*pl[=a]n"), v. i. [imp. & p. p.
   Complained (k[o^]m*pl[=a]nd"); p. pr. & vb. n.
   Complaining.] [F. complaindre, LL. complangere; com- + L.
   plangere to strike, beat, to beat the breast or head as a
   sign of grief, to lament. See Plaint.]
   1. To give utterance to expression of grief, pain, censure,
      regret. etc.; to lament; to murmur; to find fault; --
      commonly used with of. Also, to creak or squeak, as a
      timber or wheel.
      [1913 Webster]

            O loss of sight, of thee I most complain! --Milton.
      [1913 Webster]

   2. To make a formal accusation; to make a charge.
      [1913 Webster]

            Now, Master Shallow, you'll complain of me to the
            king?                                 --Shak.

   Syn: To repine; grumble; deplore; bewail; grieve; mourn;
        regret; murmur.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

complaining \complaining\ (k[o^]m*pl[=a]n"[i^]mg) adj.
   uttering complaints. Opposite of uncomplaining.
   [prenominal]

   Note: [Narrower terms: faultfinding, grumbling(prenominal):
         fretful, querulous, whiney, whining(prenominal),
         whiny; protesting(prenominal), protestant]

   Syn: complaintive.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

complaining
    adj 1: expressing pain or dissatisfaction of resentment; "a
           complaining boss" [syn: complaining(a), complaintive]
           [ant: uncomplaining]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "complaining":
   Jeremianic, beef, beefing, bellyache, bellyaching, bitch, bitching,
   complaint, complaintful, crabbing, crabby, cranky, croaking,
   destructive criticism, disappointed, discontented, disgruntled,
   displeased, disputatious, disputing, dissatisfied, dissent,
   dissentient, dissenting, envious, faultfinding, fractious, fretful,
   grievance, gripe, griping, groan, groaning, grouchy, grouse,
   grousing, growling, grumbling, holler, howl, howling, kick,
   kicking, lamentive, malcontent, malcontented, moanful, mournful,
   murmuring, muttering, noncooperative, objecting, obstructive,
   on the barricades, out of humor, peeve, peevish, peevishness,
   pet peeve, petulance, petulant, plaintive, plangent, proof against,
   protest, protesting, puling, querulant, querulous, querulousness,
   rebellious, recalcitrant, refractory, reluctant, renitent,
   repellent, resentful, resistant, resisting, resistive, restive,
   restless, retardant, retardative, scolding, sniping, sorrowful,
   squawk, squawking, sulky, ululant, unaccepting, unaccommodating,
   uncooperative, uneasy, unfulfilled, ungratified, unhappy,
   unsatisfied, unsubmissive, unyielding, up in arms, wailful,
   whimpering, whining, whiny, withstanding, yapping