OpenID. Your visitors can authenticate with their OpenID to see privileged content on your site.
I was very intrigued with how they provided this support based on my desire to see OpenID used specifically for this purpose. I have a previous blog entry on the topic here. Basically, Chi.mp uses the concept of "Personas" to segregate content in any way the user chooses. Then contacts are assigned to specific "Personas". As yet, I have not been able to find a way to assign a contact to more than one Persona but that doesn't really affect functionality.
Another very nice feature of their implementation is that if a user goes to my public profile page and logs in with their OpenID, I'm notified that the user has attempted to connect with me. This is a very nice interface as Chi.mp uses SREG support to collect information about the user. Now I can review the "connect" requests and had them as contacts assigning them to a Persona (if I choose). You can try this out with my Chi.mp profile here :)
The only problem I ran into was trying to add a contact via the web page because there was no place for me to enter that contact's OpenID. I'm sure this can be added easily.
Props to Chi.mp for moving beyond "security by obscurity" when it comes to protected sharing of personal information, and also not requiring all my friends to create a Chi.mp account.