To correct Bob Brooks, there IS indeed a Manhattan MLS. However as it is in it's infancy, there are perhaps about 40 companies associated with it. The main board in Manhattan is the Real Estate board of NY, or REBNY. Almost all brokerages are associated with them (300+ firms and counting!) and many firms that are in the MLS are also members of REBNY.
If you are a customer with serious intent, it would be to your advantage to contact some brokers and work with someone you feel comfortable with. They can provide you with more than just door openings. Good luck.