ADFS or Active Directory Federation Services enables web single sign-on, federated identity and access management by securely sharing digital identity and entitlement rights across security and enterprise boundaries. I kind of think of it as the next generation of .NET Passport and I felt back in Redmond that this is going to be an important technology in the next coming years. Good news is that WSS and SPS 2003 both support it now if you want to try it out. It is still tricky but here is the support article to get you started. Another great resource is the following episode of the MSDN show. Thanks Michael to remind me of this.