The Collaborative International Dictionary of English v.0.48:

Instigator \In"sti*ga`tor\, n. [L.: cf. F. instigateur.]
   One who instigates or incites. --Burke.

   Syn: inciter, instigant.
        [1913 Webster]

WordNet (r) 3.0 (2006):

instigator
    n 1: someone who deliberately foments trouble; "she was the
         instigator of their quarrel" [syn: instigator,
         provoker, inciter, instigant, firebrand]
    2: a person who initiates a course of action [syn: instigator,
       initiator]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "instigator":
   agent, agent provocateur, agitator, agitprop, ancestors,
   apprentice, architect, artificer, artist, author, begetter,
   beginner, builder, catalyst, causer, conceiver, constructor,
   craftsman, creator, demagogue, designer, deviser, discoverer,
   effector, engenderer, engineer, exciter, executor, executrix,
   father, firebrand, fomenter, founder, generator, grower,
   inaugurator, incendiary, inciter, industrialist, inflamer,
   initiator, inspirer, institutor, introducer, inventor, journeyman,
   maker, manufacturer, master, master craftsman, mischief-maker,
   mother, mover, organizer, originator, parent, past master, planner,
   precursor, prime mover, primum mobile, producer, provocateur,
   provoker, rabble-rouser, raiser, realizer, ringleader, rouser,
   seditionary, seditionist, shaper, sire, smith, troublemaker, urger,
   wright