The Collaborative International Dictionary of English v.0.48:

Unnecessary \Un*nec"es*sa*ry\, a.
   Not necessary; not required under the circumstances; unless;
   needless; as, unnecessary labor, care, or rigor. --
   Un*nec"es*sa*ri*ly, adv. -- Un*nec"es*sa*ri*ness, n.
   [1913 Webster]
   [1913 Webster]

WordNet (r) 3.0 (2006):

unnecessary
    adj 1: not necessary [syn: unnecessary, unneeded] [ant:
           necessary]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "unnecessary":
   de trop, dispensable, disposable, excess, expendable, expletive,
   gratuitous, in excess, inessential, needless, nonessential,
   pleonastic, prolix, redundant, spare, supererogatory, superfluous,
   surplus, tautologic, tautologous, to spare, uncalled-for,
   unessential, unneeded, unrequired, unwanted, verbose