General:
* Intermediary
* Mediator
* Agent
* Facilitator
* Go-between
* Negotiator
Financial:
* Dealer
* Trader
* Merchant
* Speculator
* Financial advisor
* Investment banker
* Stockbroker
Other:
* Matchmaker (for relationships)
* Middleman
* Go-between
* Representative
* Liaison
* Arranger
* Dealmaker
The best synonym to use will depend on the specific context in which you're using the word "broker."