As a sinner but devout follower of the Church of Tufte and general fan of information display and the like, I’ve been following the work of the Simile group at MIT for a while. Their Exhibit and Timeline projects are really interesting takes on complex visualization problems, well done.
Meet the latest work product from that group, Seek. Seek is a Thunderbird 2 extension (doesn’t work without modification on trunk nightlies yet) from David François Huynh to do faceted email browsing. It even includes a timeline visualization component. Check out the screencast, or just install the extension to give it a shot and play with it. This is experimental, early stage work, but I think it’s superb — it should I think spark all kinds of great discussions as to new ways of mining this incredibly rich data store we call email archives.