The Collaborative International Dictionary of English v.0.48:

Conspirator \Con*spir"a*tor\, n.
   One who engages in a conspiracy; a plotter. --2 Sam. xv. 31.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conspirator
    n 1: a member of a conspiracy [syn: conspirator,
         coconspirator, plotter, machinator]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "conspirator":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Machiavellian,
   Quisling, abettor, accessory, accomplice, archtraitor, betrayer,
   cockatrice, coconspirator, conniver, conspirer, counterplotter,
   double agent, double-crosser, double-dealer, exploiter, finagler,
   informer, intrigant, intriguer, machinator, maneuverer, operator,
   opportunist, plotter, quisling, rat, schemer, serpent, snake,
   timeserver, traitor, treasonist, trimmer, turncoat, wire-puller