The goal of Indigo is to simplify the process of building distributed applications, where software components communicate across a network using Web services protocols. For example, the Indigo communications system will allow an application written with Microsoft's .Net tools to share information with a Java application without the need for special code to bridge the two systems.More details about Indigo and Longhorn at Cnet
Microsoft presents Indigo technology preview

Microsoft released a technology preview of Indigo to its MSDN subscribers. Indigo is a new communications system to let Windows program communicate more easily with other software.