The Collaborative International Dictionary of English v.0.48:

nonpartisan \nonpartisan\ adj.
   1. free from party affiliation or bias. Opposite of
      partisan. [Narrower terms: bipartisan, bipartizan,
      two-party, two-way; independent ] [Narrower terms:
      {nonparty, non-party ]

   Syn: nonpartizan.
        [WordNet 1.5]

   2. not affiliated with any one party; as, a nonpartisan
      commission to study crime.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

nonpartisan
    adj 1: free from party affiliation or bias [syn: {nonpartisan,
           nonpartizan] [ant: partisan, partizan]
    n 1: a person who is nonpartisan [syn: nonpartisan,
         nonpartizan] [ant: drumbeater, partisan, zealot]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "nonpartisan":
   Laodicean, anythingarian, apathetic, autarchic, autarkic,
   autonomous, centrist, equitable, even, fence-sitter, fifty-fifty,
   free lance, free spirit, free trader, free-spirited, freethinker,
   freewheeling, half-and-half, impartial, independent, indifferent,
   individualist, individualistic, inner-directed, isolationist, just,
   latitudinarian, liberal, libertarian, libertine, midway, moderate,
   mugwump, mugwumpian, mugwumpish, neuter, neutral, nonaligned,
   noncommitted, nothingarian, objective, on the fence, passive,
   rugged individualist, self-contained, self-dependent,
   self-determined, self-directing, self-governed, self-governing,
   self-reliant, self-subsistent, self-sufficient, self-supporting,
   sovereign, swing vote, third force, third world, third-force,
   third-world, uncolored, uncommitted, uncommitted voter, uninvolved,
   unprejudiced