The Collaborative International Dictionary of English v.0.48:

Extremist \Ex*trem"ist\, n.
   A supporter of extreme doctrines or practice; one who holds
   extreme opinions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extremist
    adj 1: (used of opinions and actions) far beyond the norm;
           "extremist political views"; "radical opinions on
           education"; "an ultra conservative" [syn: extremist,
           radical, ultra]
    n 1: a person who holds extreme views

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "extremist":
   Bolshevik, Fabian, Jacobin, Wobbly, agitator, anarchist,
   anarchistic, anarcho-syndicalist, bigoted, brawler, extravagant,
   extreme, extreme left-winger, extremistic, fanatic, frondeur,
   haggard, inordinate, insubordinate, insurgent, insurrectionary,
   insurrectionist, insurrecto, irrational, left-wing extremist,
   lunatic fringe, malcontent, maverick, meliorist, mild radical,
   mildly radical, mutineer, nihilist, nihilistic, nonconformist,
   overenthusiastic, overreligious, overzealous, parlor Bolshevik,
   parlor pink, perfervid, pink, pinko, progressionist, progressist,
   progressive, progressivist, rabid, radical, rebel, red, reformer,
   reformist, revisionist, revolter, revolutional, revolutionary,
   revolutionist, rioter, sans-culotte, subversive, syndicalist,
   traitor, ultra, ultraconservative, ultraist, ultraistic,
   ultrazealous, unreasonable, utopian, utopist, wild-eyed,
   wild-looking, yippie, zealotic