General:
* Broker
* Agent
* Facilitator
* Middleman
* Go-between
* Intermediary
* Connector
* Matchmaker
Specific to certain industries:
* Wholesaler (for goods)
* Distributor (for goods)
* Realtor (for real estate)
* Insurance broker (for insurance)
* Investment banker (for financial transactions)
* Headhunter (for recruitment)
More informal:
* Gatekeeper
* Connector
* Link
* Bridge
The best word to use depends on the specific situation and the nature of the intermediary's role.