The term Enterprise Service Bus (ESB) is widely used in the context of implementing the messaging capabilities of a service oriented infrastructure. An ESB is one of many building blocks that make up a comprehensive service oriented infrastructure.  Microsoft provides a comprehensive ESB offering through its large network of partners. 

You can find a list of the partners in the early access program on the Microsoft SOA/ESB Web site:  http://www.microsoft.com/biztalk/solutions/soa/esb.mspx

Regards,

Marjan