WordNet (r) 3.0 (2006):

loner
    n 1: a person who avoids the company or assistance of others
         [syn: loner, lone wolf, lone hand]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "loner":
   Diogenes, Hieronymian, Hieronymite, Timon of Athens, alien,
   anchoress, anchorite, ascetic, bedridden invalid, careerist, case,
   character, cloistered monk, closet cynic, crackpot, crank,
   desert fathers, desert saints, eccentric, egotist, eremite,
   fanatic, fortune hunter, hermit, hermitess, hobo, hog, homebody,
   individualist, invalid, isolationist, kook, lone wolf, marabout,
   maverick, meshuggenah, monopolist, narcissist, natural,
   nonconformist, nut, odd fellow, oddball, oddity, original, outcast,
   outsider, pariah, pillar saint, pillarist, queer duck, queer fish,
   queer specimen, rara avis, recluse, road hog, screwball,
   seclusionist, self-advancer, self-pleaser, self-seeker,
   self-server, shut-in, solitaire, solitary, solitudinarian,
   stay-at-home, stylite, temporizer, timepleaser, timeserver, tramp,
   tufthunter, type, zealot