Researchers at the Sociable Media Group, in the MIT Media Lab have a project going to visualize email archives as a growing mountain:

Mountain visualizes a person's email archive in terms of all the people with whom this person has been in touch over the years. Each layer in the Mountain represents a different person. Layers are ordered by time, with the first people in the email archive at the bottom and the most recent people in the archive at the top right portion of the mountain. The thickness of each layer refers to how recently the person has been in contact with ego (the owner of the email archive). Users can highlight specific layers in the Mountain causing the first words of every email exchanged with this person to appear on the screen.

Via: Smart Mobs