WordNet (r) 3.0 (2006):

snoop
    n 1: a spy who makes uninvited inquiries into the private
         affairs of others [syn: snoop, snooper]
    v 1: watch, observe, or inquire secretly [syn: spy, stag,
         snoop, sleuth]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "snoop":
   Paul Pry, Paul-Pry, Peeping Tom, audient, auditor, backseat driver,
   busybody, butt-in, buttinsky, eavesdropper, fool with, gossip,
   hearer, hearkener, inquirer, inquisitive, inquisitor, interfere,
   intermeddle, intermeddler, intrude, intruder, kibitz, kibitzer,
   listener, meddle, meddle with, meddler, mess with, monkey with,
   mouse, nose, nose around, nosy Parker, overhearer, peek, peep,
   peeper, peer, poke, pragmatist, prier, private detective,
   private eye, private investigator, pry, querier, querist,
   questioner, quidnunc, rubberneck, rubbernecker, scopophiliac,
   shamus, sightseer, snook, snooper, spy, stare, tamper with, voyeur,
   yenta