The Collaborative International Dictionary of English v.0.48:

Uncommon \Un*com"mon\, a.
   Not common; unusual; infrequent; rare; hence, remarkable;
   strange; as, an uncommon season; an uncommon degree of cold
   or heat; uncommon courage.
   [1913 Webster]

   Syn: Rare; scarce; infrequent; unwonted.
        [1913 Webster] -- Un*com"mon*ly, adv. --
        Un*com"mon*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

uncommon
    adj 1: not common or ordinarily encountered; unusually great in
           amount or remarkable in character or kind; "uncommon
           birds"; "frost and floods are uncommon during these
           months"; "doing an uncommon amount of business"; "an
           uncommon liking for money"; "he owed his greatest debt to
           his mother's uncommon character and ability" [ant:
           common]
    2: marked by an uncommon quality; especially superlative or
       extreme of its kind; "what is so rare as a day in
       June"-J.R.Lowell; "a rare skill"; "an uncommon sense of
       humor"; "she was kind to an uncommon degree" [syn: rare,
       uncommon]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "uncommon":
   almost unheard-of, appalling, astonishing, at a premium, breakaway,
   conspicuous, different, egregious, exceptional, extraordinary,
   extremely, fabulous, fantastic, few, formidable, in short supply,
   incredible, infrequent, marked, marvelous, notable, noteworthy,
   noticeable, novel, occasional, of mark, offbeat, original,
   out of print, out of season, out of stock, outstanding, rare,
   rarely, recherche, remarkable, scanty, scarce, seldom,
   seldom met with, seldom seen, signal, singular, slow, sparse,
   sporadic, strange, striking, sui generis, superior, uncommonly,
   uncustomary, undreamed-of, unexpected, unfamiliar, unfrequent,
   unheard-of, unimaginable, unique, unordinary, unthinkable,
   unthought-of, unusual, unusually, unwonted, wonderful