The Collaborative International Dictionary of English v.0.48:

Intermediator \In`ter*me"di*a`tor\, n.
   A mediator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intermediator
    n 1: a negotiator who acts as a link between parties [syn:
         mediator, go-between, intermediator, intermediary,
         intercessor]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "intermediator":
   agent, connection, deputy, entrepreneur, front, front man,
   go-between, interagent, interceder, intercessor, intermediary,
   intermediate, intermediate agent, intermedium, internuncio,
   intervener, interventionist, interventor, mediator, medium,
   middleman, mouthpiece, negotiant, negotiator, negotiatress,
   negotiatrix, ombudsman, spokesman, spokeswoman