The Collaborative International Dictionary of English v.0.48:

Agitator \Ag"i*ta`tor\, n. [L.]
   1. One who agitates; one who stirs up or excites others; as,
      political reformers and agitators.
      [1913 Webster]

   2. (Eng. Hist.) One of a body of men appointed by the army,
      in Cromwell's time, to look after their interests; --
      called also adjutators. --Clarendon.
      [1913 Webster]

   3. An implement for shaking or mixing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

agitator
    n 1: one who agitates; a political troublemaker [syn:
         agitator, fomenter]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "agitator":
   activist, agent provocateur, agitprop, beater, blender, brawler,
   catalyst, cement mixer, churn, crucible, demagogue, eggbeater,
   emulsifier, exciter, extremist, firebrand, fomenter, frondeur,
   incendiary, inciter, inflamer, instigator, insubordinate,
   insurgent, insurrectionary, insurrectionist, insurrecto, jiggler,
   malcontent, maverick, melting pot, mischief-maker, mixer, mover,
   mutineer, nonconformist, paddle, provocateur, provoker,
   rabble-rouser, rebel, revolter, revolutionary, revolutionist,
   ringleader, rioter, rouser, seditionary, seditionist, shaker,
   subversive, traitor, troublemaker, urger, vibrator, whisk



The Devil's Dictionary (1881-1906):

AGITATOR, n.  A statesman who shakes the fruit trees of his neighbors
-- to dislodge the worms.