The Collaborative International Dictionary of English v.0.48:

Unpopular \Unpopular\
   See popular.

WordNet (r) 3.0 (2006):

unpopular
    adj 1: regarded with disfavor or lacking general approval;
           "unpopular ideas"; "an unpopular war" [ant: popular]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "unpopular":
   avoided, despised, disliked, friendless, ignored, ignotus,
   in bad odor, inglorious, little known, lowly, misprized,
   misunderstood, nameless, no credit to, obscure, out of favor,
   rejected, renownless, shunned, snubbed, thankless, unaccepted,
   unappreciated, uncared-for, uncelebrated, undesirable,
   undistinguished, unfamed, unglorified, unheard-of, unhonored,
   unknown, unlamented, unliked, unloved, unmissed, unmourned,
   unnotable, unnoted, unnoticed, unregretted, unremarked, unrenowned,
   unsung, unvalued, unwanted, unwelcome