The Collaborative International Dictionary of English v.0.48:

Unlike \Un*like"\, a.
   1. Not like; dissimilar; diverse; having no resemblance; as,
      the cases are unlike.
      [1913 Webster]

   2. Not likely; improbable; unlikely. [Obsoles.]
      [1913 Webster]

   Unlike quantities (Math.), quantities expressed by letters
      which are different or of different powers, as a, b, c,
      a^2, a^3, x^n, and the like.

   Unlike signs (Math.), the signs plus (+) and minus (-).
      [1913 Webster]

WordNet (r) 3.0 (2006):

unlike
    adj 1: marked by dissimilarity; "for twins they are very
           unlike"; "people are profoundly different" [syn:
           unlike, dissimilar, different] [ant: like,
           similar]
    2: not equal in amount; "they distributed unlike (or unequal)
       sums to the various charities" [ant: like, same]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "unlike":
   assorted, at odds, at variance, atypical, contrary, contrasted,
   contrasting, counter, departing, deviating, deviative, different,
   differentiated, differing, disaccordant, disagreeing, discordant,
   discrepant, discrete, discriminated, disjoined, disparate,
   dissimilar, dissonant, distant, distinct, distinguished, divergent,
   diverging, divers, diverse, diversified, hardly like,
   heterogeneous, in contrast with, in disagreement, inaccordant,
   incommensurable, incomparable, incompatible, incongruous,
   inconsistent, inconsonant, inharmonious, irreconcilable, many,
   motley, multifarious, nonuniform, odd, of different orders, off,
   offbeat, opposite to, out, poles apart, poles asunder,
   scarcely like, separate, separated, several, unalike,
   uncharacteristic, unconformable, unequal, unequal to, unidentical,
   unmatched, unresembling, unsame, unsimilar, untypical, variant,
   varied, variegated, various, varying, widely apart, worlds apart