The Collaborative International Dictionary of English v.0.48:

Go-between \Go"-be*tween`\, n.
   A negotiator who acts as a link between parties; an
   intermediate agent; a broker; a procurer; -- sometimes in a
   disparaging sense. --Shak.

   Syn: mediator, intermediator, intermediary.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

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