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.

http://www.xobni.com/ 

Here is an interview Scoble did with the Xobni founders (in 3 parts):