The Collaborative International Dictionary of English v.0.48:

Schemer \Schem"er\, n.
   One who forms schemes; a projector; esp., a plotter; an
   intriguer.
   [1913 Webster]

         Schemers and confederates in guilt.      --Paley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

schemer
    n 1: a planner who draws up a personal scheme of action [syn:
         schemer, plotter]

Moby Thesaurus II by Grady Ward, 1.0:

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