This is very good for web sites that have ASP.NET Authentication (or plan to have) and have Silverlight apps.

Brad Abrams, from his blog:

"Here is what I plan to show:

1. Login\Logout
2. Save personalization settings
3. Enable custom UI based on a user's role (for example, manager or employee)
4. A custom log-in control to make the UI a bit cleaner

Source code available on the above post.