The Collaborative International Dictionary of English v.0.48:

Plagiarist \Pla"gia*rist\, n.
   One who plagiarizes, or purloins the words, writings, or
   ideas of another, and passes them off as his own; a literary
   thief; a plagiary.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plagiarist
    n 1: someone who uses another person's words or ideas as if they
         were his own [syn: plagiarist, plagiarizer,
         plagiariser, literary pirate, pirate]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "plagiarist":
   Artful Dodger, Casanova, Don Juan, Machiavel, Machiavelli,
   Machiavellian, actor, ape, bamboozler, befuddler, beguiler,
   charmer, conformist, copier, copycat, copyist, copyright infringer,
   counterfeiter, cribber, cuckoo, deceiver, deluder, dissembler,
   dissimulator, dodger, double-dealer, duper, echo, echoer, echoist,
   enchanter, entrancer, faker, fooler, forger, gay deceiver, hoaxer,
   hypnotizer, hypocrite, imitator, impersonator, impostor, jilt,
   jilter, joker, jokester, kidder, leg-puller, literary pirate,
   mesmerizer, mime, mimer, mimic, mimicker, misleader, mocker,
   mockingbird, monkey, parrot, phony, pirate, plagiarizer, playactor,
   poll-parrot, polly, polly-parrot, poseur, practical joker, ragger,
   role-player, seducer, sheep, simulator, spoofer, tease, teaser