The Collaborative International Dictionary of English v.0.48:

Monologist \Mo*nol"o*gist\, n. [See Monologue.]
   One who soliloquizes; esp., one who monopolizes conversation
   in company. --De Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monologist
    n 1: an entertainer who performs alone

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "monologist":
   Hamlet, Roscius, actor, actress, antagonist, bad guy, barnstormer,
   character, character actor, character man, character woman,
   child actor, diseur, diseuse, dramatizer, feeder, foil, heavy,
   histrio, histrion, ingenue, juvenile, matinee idol, mime, mimer,
   mimic, mummer, pantomime, pantomimist, playactor, player,
   protean actor, reciter, soliloquist, soliloquizer, soubrette,
   stage performer, stage player, stooge, straight man, stroller,
   strolling player, theatrical, thespian, trouper, utility man,
   villain