The Collaborative International Dictionary of English v.0.48:

crackpot \crackpot\ n.
   a whimsically eccentric person.

   Syn: crank, nut, nutcase, screwball.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

crackpot
    n 1: a whimsically eccentric person [syn: crackpot, crank,
         nut, nut case, fruitcake, screwball]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "crackpot":
   alien, aliene, bedlamite, borderline case, case, character,
   crackbrain, crank, cuckoo, dement, demoniac, ding-a-ling, dingbat,
   eccentric, energumen, erratic, fanatic, flake, fou, freak,
   harebrain, hermit, hobo, idiot, kook, 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