Plaxo is a social networking tool that allows you to dynamically update your address book. I've written a couple of posts on my personal blog about how the tool can be used.
It recently started allowing synching from LinkedIN. It would be interesting if it allowed for additional synchs from other social networking sites like SecondLife. Increasingly people are using SecondLife avatars to make connections (business or otherwise) and as tools like LinkedIN and Plaxo continue to be open to one another I think we will start to see a blending of tools where we use the best tool for a certain purpose to access the same information.