The Collaborative International Dictionary of English v.0.48:

Imitator \Im"i*ta"tor\, n. [L.]
   One who imitates.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imitator
    n 1: someone who (fraudulently) assumes the appearance of
         another [syn: impersonator, imitator]
    2: someone who copies the words or behavior of another [syn:
       copycat, imitator, emulator, ape, aper]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "imitator":
   ape, conformist, copier, copycat, copyist, counterfeiter, cuckoo,
   dissembler, dissimulator, echo, echoer, echoist, faker, forger,
   hypocrite, impersonator, impostor, mime, mimer, mimic, mimicker,
   mocker, mockingbird, monkey, parrot, phony, plagiarist,
   poll-parrot, polly, polly-parrot, poseur, sheep, simulator