WordNet (r) 3.0 (2006):

atonal
    adj 1: characterized by avoidance of traditional western
           tonality [syn: atonal, unkeyed] [ant: tonal]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "atonal":
   absonant, cacophonous, cracked, diaphonic, disconsonant,
   discordant, disharmonic, disharmonious, dissonant, flat, grating,
   harsh, immelodious, inharmonic, inharmonious, musicless,
   nonmelodious, off, off-key, off-tone, out of pitch, out of tone,
   out of tune, raucous, sharp, shrill, sour, strident, tuneless,
   unharmonious, unmelodious, unmusical, untunable, untuned,
   untuneful