Federation Solutions
Microsoft's Active Directory Federation Services (ADFS), is such a solution. It offers an organization already using Active Directory an out-of-the-box, standards-based technology for collaborating with other organizations. Users in one organization can be provided with access to web-based resources at another organization, (or different security realm in the same organization), without requiring a separate account.
At a very high level, therefore, ADFS can be seen as a single sign-on technology that can be used to authenticate a user to multiple Web applications over the course of a single session.
OCG provide Federation solutions based on ADFS, which can interoperate with other technologies, such as Shibboleth.