WordNet (r) 3.0 (2006):

undreamed of
    adj 1: not imagined even in a dream [syn: undreamed,
           undreamed of, undreamt, undreamt of, unimagined]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "undreamed of":
   breakaway, imageless, offbeat, rare, recherche, sui generis,
   uncommon, unconceived, unconceptualized, unconsidered, uncustomary,
   unexpected, unfamiliar, unheard-of, unimaged, unimagined, unique,
   unordinary, unthought-of, unusual, unwonted