Active Directory Application Mode (ADAM)
Ever wanted to write AD-enabled apps and test them on Windows XP?
ADAM allows developers to package their own AD-robust repository and authentication system with their applications. Here's what MS says:
Active Directory Application Mode (ADAM) is a part of Microsoft’s fully integrated directory services available with Windows Server 2003, and is built specifically to address directory-enabled application scenarios. ADAM runs as a non-operating-system service, and, as such, it does not require deployment on a domain controller. Running as a non-operating-system service means that multiple instances of ADAM can run concurrently on a single server, and each instance can be configured independently.
Active Directory Application Mode represents a breakthrough in directory services technology that provides flexibility, and helps organizations avoid increased infrastructure costs.
ADAM is still in beta form, but you can download it right off of Microsoft's site. I would highly recommend getting started building ASP.NET apps that use ADAM for authentication today.