The Microsoft ASP.NET v1.1 Membership Management Component Prototype contains classes that allow a developer to more easily authenticate users, authorize users, and store per-user property data in a user profile. The authentication feature validates and stores user credentials which a developer can use to manage user authentication on a web site. The authorization feature lets you treat groups of users as a unit by assigning users to roles such as manager, sales, member, and so on. Combined with ASP.NET's built-in authorization functionality, Windows Shared Hosting developers have end-to-end support for maintaining user-to-role mappings and authorizing users based on this information. The profile feature enables you to provide users of your Web site with a custom experience. By defining and using profile properties, you can track any custom information your application requires, including user information and user preferences.
UPDATE: I was reminded by the Spider King that "There are already two applications in beta using this component; DotNetNuke by Perpetual Motion and Community Server by Telligent Systems.