The Collaborative International Dictionary of English v.0.48:

Peaceful \Peace"ful\, a.
   1. Possessing or enjoying peace; not disturbed by war,
      tumult, agitation, anxiety, or commotion; quiet; tranquil;
      as, a peaceful time; a peaceful country; a peaceful end.
      [1913 Webster]

   2. Not disposed or tending to war, tumult or agitation;
      pacific; mild; calm; peaceable; as, peaceful words.
      [1913 Webster]

   Syn: See Peaceable.
        [1913 Webster] --Peace"ful*ly, adv.. --
        Peace"ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

peaceful
    adj 1: not disturbed by strife or turmoil or war; "a peaceful
           nation"; "peaceful times"; "a far from peaceful
           Christmas"; "peaceful sleep" [syn: peaceful,
           peaceable] [ant: unpeaceful]
    2: peacefully resistant in response to injustice; "passive
       resistance" [syn: passive, peaceful]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "peaceful":
   accordant, agreeable, agreeing, akin, amicable, at one, at peace,
   at rest, attuned, bland, bloodless, calm, cheerful, cloistered,
   collected, comfortable, comfy, commodious, compatible, composed,
   concordant, congenial, constant, contented, convenient, cool,
   coolheaded, corresponding, cozy, cushioned, cushiony, cushy,
   dwindling, easeful, easy, ebbing, empathetic, empathic, en rapport,
   equable, even-tenored, frictionless, friendly, gentle, halcyon,
   harmonious, homelike, homely, homey, homish, hushed, idyllic,
   impassive, in accord, in concert, in rapport, in tune, inharmony,
   intimate, irenic, isolated, judicious, like-minded, lived-in,
   luxurious, mild, mild as milk, moderate, moldering, nonviolent,
   of one mind, orderly, pacific, pacificatory, pacifist, pacifistic,
   pastoral, peaceable, peacetime, philosophical, piping, placid,
   plain, prudent, quiescent, quiet, relaxing, reposeful, reposing,
   restful, resting, roomy, secluded, sequestered, sequestrated,
   serene, sheltered, simple, smooth, snug, sober, soft, steady,
   still, still as death, stillish, stilly, stoic, stolid, subsiding,
   sympathetic, tame, temperate, together, tranquil, unagitated,
   understanding, undisturbed, united, unmoved, unperturbed,
   unpretending, unruffled, unstirring, untroubled, waning, warm