The Collaborative International Dictionary of English v.0.48:

collaborator \col*lab"o*ra`tor\, n. [L. collaborare to labor
   together; col- + laborare to labor: cf. F. collaborateur.]
   1. An associate in labor, especially in literary or
      scientific labor.
      [1913 Webster]

   2. one who willingly cooperates with an enemy, especially an
      enemy nation occupying one's own country.
      [PJC]

WordNet (r) 3.0 (2006):

collaborator
    n 1: someone who assists in a plot [syn: confederate,
         collaborator, henchman, partner in crime]
    2: someone who collaborates with an enemy occupying force [syn:
       collaborator, collaborationist, quisling]
    3: an associate in an activity or endeavor or sphere of common
       interest; "the musician and the librettist were
       collaborators"; "sexual partners" [syn: collaborator,
       cooperator, partner, pardner]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "collaborator":
   Trojan horse, advertising writer, annalist, apostate, art critic,
   author, authoress, backslider, belletrist, bibliographer, bolter,
   coauthor, collaborationist, columnist, compiler, composer, convert,
   copywriter, creative writer, critic, crypto, dance critic,
   defector, deserter, diarist, drama critic, dramatist,
   encyclopedist, essayist, fellow traveler, fifth column,
   fifth columnist, fraternizer, free lance, free-lance writer, ghost,
   ghostwriter, humorist, inditer, literary artist,
   literary craftsman, literary critic, literary man, litterateur,
   logographer, magazine writer, man of letters, monographer, mugwump,
   music critic, newspaperman, novelettist, novelist, pamphleteer,
   penwoman, poet, prose writer, proselyte, quisling, recidivist,
   recreant, renegade, renegado, renegate, reversionist, reviewer,
   runagate, saboteur, scenario writer, scenarist, schismatic, scribe,
   scriptwriter, seceder, secessionist, security risk, separatist,
   short-story writer, storyteller, strikebreaker, subversive,
   technical writer, tergiversant, tergiversator, traitor, turnabout,
   turncoat, turntail, underground, word painter, wordsmith, writer