This article on FlowingData reviews 21 ways to visualize email traffic and contents. Some of the examples include themail, which visualizes the topics that users discuss with their contacts (see paper and screenshot). Mountain visualizes email archives in terms of all the people with whom the user has been in touch over the years, whereby each layer in the Mountain represents a different person (see screenshot). Anymails uses a microbe metaphor to visualize the structure and attributes of the user's inbox (see screenshot).
