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):

peacefully
    adv 1: in a peaceful manner; "the hen settled herself on the
           nest most peacefully"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "peacefully":
   calmly, comfortably, commodiously, conveniently, cozily, easily,
   luxuriously, placidly, quiescently, quietly, reposefully,
   restfully, roomily, smoothly, snugly, still, stilly, tranquilly,
   unperturbedly, voluptuously, with ease