The Collaborative International Dictionary of English v.0.48:

Beautiful \Beau"ti*ful\ (b[=u]"t[i^]*f[.u]l), a.
   Having the qualities which constitute beauty; pleasing to the
   sight or the mind.
   [1913 Webster]

         A circle is more beautiful than a square; a square is
         more beautiful than a parallelogram.     --Lord Kames.
   [1913 Webster]

   Syn: Handsome; elegant; lovely; fair; charming; graceful;
        pretty; delightful. See Fine. -- Beau"ti*ful*ly,
        adv. -- Beau"ti*ful*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

beautiful
    adj 1: delighting the senses or exciting intellectual or
           emotional admiration; "a beautiful child"; "beautiful
           country"; "a beautiful painting"; "a beautiful theory";
           "a beautiful party" [ant: ugly]
    2: (of weather) highly enjoyable; "what a beautiful day"

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "beautiful":
   admirable, aesthetic, aesthetically appealing, alluring, appealing,
   art-conscious, artistic, arty, attractive, beauteous, bonny,
   charming, choice, comely, delightful, elegant, endowed with beauty,
   excellent, exquisite, eye-filling, fair, fine, first-rate,
   flowerlike, glorious, good-looking, gorgeous, graceful, gracile,
   handsome, hear, heavy, incomparable, lovely, magnificent,
   of consummate art, ornamental, painterly, personable, pleasant,
   pleasing, pretty, proper, pulchritudinous, radiant, resplendent,
   smashing, spectacular, splendid, stunning, sublime, superb,
   superior, tasteful, well done, well-favored, wonderful