The Collaborative International Dictionary of English v.0.48:

Commentator \Com"men*ta`tor\, n. [L. commentator: cf. F.
   commentateur.]
   One who writes a commentary or comments; an expositor; an
   annotator.
   [1913 Webster]

         The commentator's professed object is to explain, to
         enforce, to illustrate doctrines claimed as true.
                                                  --Whewell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commentator
    n 1: an expert who observes and comments on something [syn:
         observer, commentator]
    2: a writer who reports and analyzes events of the day [syn:
       commentator, reviewer]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "commentator":
   AFTRA, DJ, MC, allegorist, anchor man, annotator, announcer,
   book reviewer, broadcaster, captious critic, carper, caviler,
   censor, censurer, cicerone, clarifier, cognoscente, commenter,
   connoisseur, critic, criticaster, criticizer, critickin, criticule,
   cryptanalyst, cryptographer, cryptologist, decoder, definer,
   demonstrator, demythologizer, diaskeuast, disk jockey, dragoman,
   editor, editorial writer, editorialist, emcee, emendator, emender,
   euhemerist, exegesist, exegete, exegetist, explainer, explicator,
   exponent, expositor, expounder, faultfinder, glossarist,
   glossographer, go-between, guide, hermeneut, interpreter,
   leader writer, lexicographer, literary critic, man of letters,
   master of ceremonies, metaphrast, muckraker, news analyst,
   news commentator, newscaster, oneirocritic, paraphrast,
   program director, programmer, publicist, radiobroadcaster,
   reviewer, scholiast, smellfungus, social critic, sound-effects man,
   sportscaster, textual critic, translator