WordNet (r) 3.0 (2006):

Peeping Tom
    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:

21 Moby Thesaurus words for "Peeping Tom":
   Paul Pry, busybody, eavesdropper, gossip, inquirer, inquisitive,
   inquisitor, nosy Parker, pry, querier, querist, questioner,
   quidnunc, rubberneck, rubbernecker, scopophiliac, sightseer, snoop,
   snooper, voyeur, yenta