The Collaborative International Dictionary of English v.0.48:

Protester \Pro*test"er\, n.
   1. One who protests; one who utters a solemn declaration.
      --Shak.
      [1913 Webster]

   2. (Law) One who protests a bill of exchange, or note.
      [1913 Webster]

WordNet (r) 3.0 (2006):

protester
    n 1: a person who dissents from some established policy [syn:
         dissenter, dissident, protester, objector,
         contestant]
    2: someone who participates in a public display of group feeling
       [syn: demonstrator, protester]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "protester":
   apostate, bitter-ender, brawler, defendant, demurrer, diehard,
   disputant, dissenter, dissentient, dissident, intransigent,
   irreconcilable, last-ditcher, litigant, naysayer, negativist,
   nonconformist, noncooperator, objector, obstructionist,
   obstructive, opinionist, opposer, opposition voice, oppositionist,
   plaintiff, protestant, quarreler, recusant, resister, schismatic,
   scrapper, sectarian, sectary, separatist, wrangler