ISVs and Hosters can use the MPS SDK to develop applications that are enabled for multi-tenancy capability.
This SDK helps to create resource providers and namespaces specific to provisioning and hosting that allow your applications to be used by many users simultaneously.

The MPS SDK requires MPS, a server product and development framework included in Microsoft Solutions for Windows based Hosting. MPS is developed at Microsoft and can be used to provision any kind of software services - from the creation of a user inside Active Directory, to the enablement of hosted extranet services for an enterprise, including messaging, collaboration and directory synchronization.

MPS runs with a list of resource managers (Active Directory, Internet Information Server, etc)
, and can be customized for the needs of hosted solutions architects and developers, by developing specific resources managers with this SDK.

More about MPS? see this entry point on hosting solutions here on microsoft.com web site. Also, it has it's own forum on asp.net, here. And you can see MPS live through a web console running in demo mode here
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MPS SDK is a free download available since Oct 17th, 2006 at this URL.