WordNet (r) 3.0 (2006):

alter ego
    n 1: a very close and trusted friend who seems almost a part of
         yourself

Moby Thesaurus II by Grady Ward, 1.0:

136 Moby Thesaurus words for "alter ego":
   I, I myself, acquaintance, advocate, ally, alter, alternate,
   alterum, amicus curiae, analogon, analogue, associate, attorney,
   backer, backup, backup man, best friend, better self, bosom friend,
   brother, casual acquaintance, champion, close acquaintance,
   close copy, close friend, close match, cognate, companion,
   complement, confidant, confidante, congenator, congener,
   coordinate, correlate, correlative, correspondent, counterpart,
   deputy, dummy, ego, equivalent, ethical self, executive officer,
   exponent, familiar, favorer, fellow, fellow creature, fellowman,
   fidus Achates, figurehead, friend, he, her, herself, him, himself,
   image, inner man, inner self, inseparable friend, intimate, it,
   kindred spirit, lieutenant, like, likeness, locum, locum tenens,
   lover, man Friday, mate, me, my humble self, myself,
   near duplicate, neighbor, number one, obverse, oneself, other self,
   ourselves, parallel, paranymph, partisan, pendant, pickup, picture,
   pinch hitter, pleader, procurator, proxy, reciprocal, repository,
   representative, right hand, right-hand man, second in command,
   second self, secondary, self, she, similitude, simulacrum, sister,
   soul mate, stand-in, strong right hand, subconscious self,
   subliminal self, substitute, such, suchlike, superego, supporter,
   surrogate, sympathizer, tally, the like of, the likes of, them,
   themselves, they, twin, understudy, utility man, vicar,
   vicar general, vice, vicegerent, well-wisher, you, yours truly,
   yourself, yourselves