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 Cèsar 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.
