An interesting visualization tool has been created by a team at MIT called Immersion. It maps your network of contacts by analyzing the metadata in your Gmail account. It doesn't read your emails per se, but just some of the metadata. (Coincidentally, that's what the NSA is reading from your emails as well!)
See the sample image below from their demo.
I have about 7 years worth of emails in my Gmail account, so it provides an interesting picture of my contacts. I don't use Gmail for all of my business communications -- once I'm on contract with a client, they usually issue me a company email account -- however, it still gives a clear picture of what parts of my network are connected and which clusters are distinct.
Not sure I can do anything useful with this knowledge, but it was an interesting visualization to see. At least it made it worth keeping all those old emails that I felt I should've cleaned up long ago.
Procrastination has its benefits!