I’ve been using Xobni for a couple of months now and I am totally in love with it. It lets me do things with my email data that I just could not do otherwise. A simple example: last week I was trying to find someone whom I only knew two things about: first name and involvement in a certain community. With Xobni I was able to find the person. Turns out he and I had both been recipients of some of the same emails.
Here is an interview Scoble did with the Xobni founders (in 3 parts):

