The Collaborative International Dictionary of English v.0.48:

Harmonious \Har*mo"ni*ous\ (h[aum]r*m[=o]"n[i^]*[u^]s), a. [Cf.
   F. harmonieux. See Harmony.]
   1. Adapted to each other; having parts proportioned to each
      other; symmetrical.
      [1913 Webster]

            God hath made the intellectual world harmonious and
            beautiful without us.                 --Locke.
      [1913 Webster]

   2. Acting together to a common end; agreeing in action or
      feeling; living in peace and friendship; as, an harmonious
      family.
      [1913 Webster]

   3. Vocally or musically concordant; agreeably consonant;
      symphonious. -- Har*mo"ni*ous*ly, adv. --
      Har*mo"ni*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

harmonious
    adj 1: musically pleasing [ant: inharmonious, unharmonious]
    2: exhibiting equivalence or correspondence among constituents
       of an entity or between different entities [syn:
       harmonious, proportionate, symmetrical]
    3: suitable and fitting; "the tailored clothes were harmonious
       with her military bearing"
    4: existing together in harmony; "harmonious family
       relationships"

Moby Thesaurus II by Grady Ward, 1.0:

280 Moby Thesaurus words for "harmonious":
   accompanying, accordant, according, affable, affirmative,
   agreeable, agreeing, akin, amiable, amicable, anal, answerable,
   arranged, associate, associated, assonant, assonantal, at one,
   at one with, attuned, balanced, blended, blending, blissful,
   bourgeois, brotherly, businesslike, canorous, cheerful, chiming,
   chromatic, coacting, coactive, coadjutant, coadjuvant, coadunate,
   coefficient, coequal, coexistent, coexisting, coherent, coincident,
   coinciding, cold, collaborative, collective, collectivist,
   collectivistic, collusive, colorific, coloring, combined,
   combining, commensal, commensurate, common, communal, communalist,
   communalistic, communist, communistic, communitarian, compatible,
   complaisant, complementary, compulsive, concerted, concinnate,
   concinnous, concomitant, concordant, concurrent, concurring,
   conformable, conformist, congenial, congruent, congruous, conjoint,
   conniving, consentaneous, consentient, consilient, consistent,
   consonant, conspiratorial, contrapuntal, conventional, cool,
   cooperant, cooperating, cooperative, coordinate, cordial,
   correspondent, corresponding, coworking, desirable, dichromatic,
   dulcet, ecumenic, empathetic, empathic, en rapport, enjoyable,
   equal, equilateral, equivalent, euphonic, euphonical, euphonious,
   eurythmic, even, fair, fair and pleasant, favorable, felicific,
   felicitous, fellow, fine, finished, flowing, fluent, formal,
   formalistic, fraternal, frictionless, friendlike, friendly, genial,
   glowing, good, goodly, gracious, grateful, gratifying, habitual,
   harmonic, harmonized, harmonizing, heart-warming, homophonic,
   honeyed, in accord, in agreement, in chorus, in concert,
   in concord, in hand, in keeping, in line, in rapport, in step,
   in sync, in synchronization, in tune, in unison, inaccordance,
   inharmony, joint, kind, kosher, likable, like-minded, many-colored,
   matching, measured, medley, meeting, mellifluous, mellisonant,
   mellow, melodious, methodical, monochromatic, monochrome,
   monochromic, monodic, monophonic, motley, musical, mutual,
   neighborlike, neighborly, nice, noncompetitive, normal, of a piece,
   of like mind, of one accord, of one mind, on all fours, ordered,
   orderly, orthodox, parasitic, parti-colored, peaceable, peaceful,
   pedantic, pigmentary, plastic, pleasant, pleasing, pleasurable,
   pleasure-giving, pleasureful, polychromatic, polyphonic, positive,
   precisianistic, prismatic, proportionate, proportioned, rainbow,
   reciprocal, reconcilable, regular, rewarding, routine, saprophytic,
   satisfying, self-consistent, silvery, simpatico, sisterly, smooth,
   smooth-sounding, sociable, sonorous, spectral, square, steady,
   straight, stuffy, sweet, symbiotic, symmetric, symmetrical,
   sympathetic, sympathique, symphonic, symphonious, synchronized,
   synchronous, synergetic, synergic, synergistic, systematic,
   tinctorial, tingent, together, toning, traditionalist, tripping,
   tuned, tuneful, unanimous, uncompetitive, understanding, unhostile,
   uniform, unisonant, unisonous, united, uniting, uptight, usual,
   variegated, warm, welcome, well-affected, well-balanced,
   well-disposed, well-intentioned, well-meaning, well-meant,
   well-ordered, well-regulated, well-set