The Collaborative International Dictionary of English v.0.48:

Reformer \Re*form"er\ (r?*f?rm"?r), n.
   1. One who effects a reformation or amendment; one who labors
      for, or urges, reform; as, a reformer of manners, or of
      abuses.
      [1913 Webster]

   2. (Eccl.Hist.) One of those who commenced the reformation of
      religion in the sixteenth century, as Luther, Melanchthon,
      Zwingli, and Calvin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reformer
    n 1: a disputant who advocates reform [syn: reformer,
         reformist, crusader, social reformer, meliorist]
    2: an apparatus that reforms the molecular structure of
       hydrocarbons to produce richer fuel; "a catalytic reformer"

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "reformer":
   Fabian, Tammany man, extremist, machine politician, meliorist,
   old campaigner, party boss, party chieftain, party leader, pol,
   political hack, political leader, politician,
   professional politician, progressionist, progressist, progressive,
   progressivist, radical, reform politician, reformist, revisionist,
   revolutionary, utopian, utopist, war-horse, wheelhorse