WordNet (r) 3.0 (2006):

unattractive
    adj 1: lacking beauty or charm; "as unattractive as most mining
           regions" [ant: attractive]
    2: lacking power to arouse interest; "being unemployed is a most
       unattractive prospect"
    3: not appealing to the senses; "untempting food" [syn:
       unattractive, untempting]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "unattractive":
   beautiless, bitter, blemished, blotted, cacophonic, cacophonous,
   defaced, disagreeable, disfigured, dislikable, displeasing,
   distasteful, dysphemistic, dysphemized, homely, inelegant, marred,
   plain, short on looks, sour, spoiled, thankless, uglified, ugly,
   ugly as hell, ugly as sin, unaesthetic, unalluring, unappealing,
   unappetizing, unbeauteous, unbeautiful, uncomely, undelectable,
   undelicious, undesirable, unengaging, unenjoyable, unhandsome,
   uninviting, unlikable, unlovely, unpalatable, unpleasant,
   unpleasing, unpretty, unsavory, unsightly, untasteful, unwelcome