Peter Gloor's TeCFlow converts a database of emails into an animated social network. This system was originally designed to handle only Eudora email format, but recently it has been upgraded to handle Outlook mailfolders as well -- broadening its appeal considerably.
I downloaded it this morning and in no time turned my Six Sigma Church email folder into this picture:
Each node is a person on the project, and each edge represents an email that I was at least cc'd on. If I got everyone else's email folders fed into TeCFlow, then the resulting picture would be of the entire system, not just my view.
Even more importantly, I'd like to get TeCFlow to recognize who the members of various mailing lists are. Right now, the graph represents each email list as an individual node, rather than showing who is really receiving those group emails.
For another well-studied perspective on visualizing email history as social networks, see Soylent, the PhD work of Danyel Fisher (formerly UC Irvine, now doing community technology at Microsoft). I heard Danyel give a great talk on Soylent at Sunbelt XXV, and only wish that Soylent supported Outlook.