WordNet (r) 3.0 (2006):

voyeur
    n 1: a viewer who enjoys seeing the sex acts or sex organs of
         others [syn: voyeur, Peeping Tom, peeper]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "voyeur":
   Paul Pry, Peeping Tom, bugger, busybody, coprophiliac,
   eavesdropper, erotomaniac, exhibitionist, fetishist, gossip,
   inquirer, inquisitive, inquisitor, masochist, narcissist,
   necrophiliac, nosy Parker, nymphomaniac, paraphiliac, pederast,
   pedophiliac, peeper, pry, querier, querist, questioner, quidnunc,
   rapist, rubberneck, rubbernecker, sadist, satyr, scopophiliac,
   scotophiliac, sightseer, snoop, snooper, sodomist, sodomite,
   transvestite, yenta, zoophiliac