The Collaborative International Dictionary of English v.0.48:

Enthusiast \En*thu"si*ast\, n. [Gr. ?: cf. F. enthousiaste.]
   One moved or actuated by enthusiasm; as:
   (a) One who imagines himself divinely inspired, or possessed
       of some special revelation; a religious madman; a
       fanatic.
   (b) One whose mind is wholly possessed and heated by what
       engages it; one who is influenced by a peculiar; fervor
       of mind; an ardent and imaginative person.
       [1913 Webster]

             Enthusiasts soon understand each other. --W.
                                                  Irving.

   Syn: Visionary; fanatic; devotee; zealot. Enthusiastic

WordNet (r) 3.0 (2006):

enthusiast
    n 1: an ardent and enthusiastic supporter of some person or
         activity [syn: enthusiast, partisan, partizan]
    2: a person having a strong liking for something [syn:
       fancier, enthusiast]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "enthusiast":
   Don Quixote, Quixote, activist, addict, adherent, admirer,
   aficionado, ball of fire, bear, beaver, big-time operator, bigot,
   booster, buff, bug, bum, bustler, busy bee, champion, collector,
   crank, daydreamer, demon, devotee, disciple, doer, dreamer,
   dreamer of dreams, eager beaver, energumen, escapist, extremist,
   faddist, fan, fanatic, fanatico, fiend, follower, freak, go-getter,
   great one for, groupie, habitue, head, hobbyist, hound,
   human dynamo, hustler, idealist, infatuate, live wire, lotus-eater,
   lover, lunatic fringe, man of action, man of deeds, maniac,
   militant, monomaniac, new broom, nut, operator, partisan,
   political activist, powerhouse, promoter, prophet, pursuer,
   rhapsodist, romancer, romantic, romanticist, seer, sucker for,
   supporter, take-charge guy, utopian, utopianist, utopianizer,
   visionary, votary, wheeler-dealer, winner, wishful thinker,
   zealot