Immersion [mit.edu] is a quite revealing visualization tool of which the NSA - or your own national security agency - can only be jealous of... Developed by MIT students Daniel Smilkov, Deepak Jagdish and CÚsar Hidalgo, Immersion generates a time-varying network visualization of all your email contacts, based on how you historically communicated with them.
Immersion is able to aggregate and analyze the "From", "To", "Cc" and "Timestamp" data of all the messages in any (authorized) Gmail, MS Exchange or Yahoo email account. It then filters out the 'collaborators' - people from whom one has received, and sent, at least 3 email messages from, and to.
As a result, people that appeared together in the headers of email messages are visually connected and placed closer together than others. A timeline slider at the bottom of the screen then allows to investigate any changes in social email connectivity over time. Individual names can be selected for further detailed exploration.
For those too paranoid to trust 3 MIT students with their personal data, there is a demo account available here.