This is a great how-to article on querying Active Directory from within SQL Server. I've written in the past about using Virtual Directory technology to query SQL data via LDAP. This is the reverse and I can envision many use-cases where this would be useful. This isn't exactly new technology, but it's a new write-up on how it's done and very easy to follow.
For example, you could use this approach to extend the information available to an application without doing any data synchronization or introducing new data sources. If the application's logon ID is the user's email address, you could query AD based on that email and get info about the user's group memberships, attributes, manager, location, etc. and have that returned to the application as if the data were stored in the local app's database.
...another useful approach to keep in your development toolbox.