WordNet (r) 3.0 (2006):

kook
    n 1: someone regarded as eccentric or crazy and standing out
         from a group [syn: kook, odd fellow, odd fish, queer
         bird, queer duck, odd man out]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "kook":
   alien, aliene, bedlamite, borderline case, case, character,
   crackbrain, crackpot, crank, cuckoo, dement, demoniac, ding-a-ling,
   eccentric, energumen, erratic, fanatic, flake, fou, freak,
   harebrain, hermit, hobo, idiot, lone wolf, loner, loon, loony,
   lunatic, madman, maniac, maverick, meshuggenah, natural, noncompos,
   nonconformist, nut, odd fellow, oddball, oddity, original,
   outsider, pariah, phrenetic, queer duck, queer fish,
   queer specimen, rara avis, raving lunatic, recluse, screwball,
   solitary, strange duck, tramp, type, weirdo, zealot



The Jargon File (version 4.4.7, 29 Dec 2003):

kook


    [Usenet; originally and more formally, net.kook] Term used to describe a
    regular poster who continually posts messages with no apparent grounding in
    reality. Different from a troll, which implies a sort of sly wink on the
    part of a poster who knows better, kooks really believe what they write, to
    the extent that they believe anything.

    The kook trademark is paranoia and grandiosity. Kooks will often build up
    elaborate imaginary support structures, fake corporations and the like, and
    continue to act as if those things are real even after their falsity has
    been documented in public.

    While they may appear harmless, and are usually filtered out by the other
    regular participants in a newsgroup of mailing list, they can still cause
    problems because the necessity for these measures is not immediately
    apparent to newcomers; there are several instances on record, for example,
    of journalists writing stories with quotes from kooks who caught them
    unaware.

    An entertaining web page chronicling the activities of many notable kooks
    can be found at http://www.crank.net/usenet.html.