WordNet (r) 3.0 (2006):

quisling
    n 1: someone who collaborates with an enemy occupying force
         [syn: collaborator, collaborationist, quisling]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "quisling":
   Benedict Arnold, Brutus, Judas, Judas Iscariot, Judas-like,
   Quisling, Trojan-horse, apostate, archtraitor, backslider,
   betrayer, betraying, bolter, cockatrice, collaborationist,
   collaborator, conniver, conspirator, conspirer, convert, convict,
   criminal, crook, deceiver, defector, deserter, desperado,
   desperate criminal, double agent, double-crosser, double-crossing,
   double-dealer, felon, fifth columnist, fifth-column, fugitive,
   gallows bird, gangster, gaolbird, informer, intrigant, intriguer,
   jailbird, lawbreaker, machinator, mobster, mugwump, outlaw,
   plotter, proselyte, public enemy, quislingistic, racketeer, rat,
   recidivist, recreant, renegade, renegado, renegate, reversionist,
   runagate, schemer, schismatic, scofflaw, seceder, secessionist,
   separatist, serpent, snake, strikebreaker, swindler, tergiversant,
   tergiversator, thief, thug, timeserver, traitor, traitorous,
   treasonable, treasonist, treasonous, trimmer, turnabout, turncoat,
   turntail, two-timer, two-timing