Outlook Graph is a windows application I've been working on to explore the connections that exist in large mail stores. If you're feeling brave, I've just released an alpha version:
It will examine all of the Outlook mail messages on the computer, and try to arrange them into a graph with related people close together. The frequency of contacts between two people shown by the darkness of the lines connecting them. My goal is to discover interesting patterns and groupings, as a laboratory for developing new tools based on email data.
The application sends no network traffic, and doesn't modify any data, but since it's still in the early stages of testing, I'd recommend using it only on fully backed-up machines. It runs a physics simulation to find a good graph, so on very large numbers of messages it may take a long time to process. I've been testing on 10-12,000 message mailboxes on my laptop, so I'll be interested in hearing how it scales up beyond that.