email constellations is an open-source tool that scans a person's mailbox and produces a graph of social groups by examining To:, From:, and Cc: headers. Apache Agora 1.1 is a similar open source tool.
NetVis is a free open-source tool for graphing relationship networks. It seems to work from tables showing the strength of relationship between the individuals being graphed.
BlogMatcher lists all blogs that link to any given blog.
InFlow maps all sorts of networks (including, for example clusters of books that are bought by the same people). The site also has some interesting articles on network mapping.
With the rise of the internet, there has been much renewed academic interest in social and other networks. Many of the links below come from the knowledge2go book (http://www.criticalmethods.org/k2g).
There is a nice, short introduction to social network analysis at orgnet.com crisply defining concepts such as degrees, betweenness, closeness, boundary spanners, peripheral players, network centralization, structural equivalence, structural holes, the E/I Ratio and small worlds. Orgnet.com also sell network mapping software. An article on Social Network Analysis in organisations here.
Marco Susani presented an interesting paper at the doors of perception conference/website on "mapping communication" in 2002 in which he identifies different patterns of group conversation and gives them names such as "the womb", "intimate daisy", "fish", "crest", and "infinite star" - each suggestive of the types of information flows occurring.
Three recent books on networking to check out:Linked by Barabasi. Nexus (pubished as Small World in the UK) by Buchanan Six Degrees: the Science of a Connected Age by Duncan Watts. There is an interview with Watts at MIT Technology Review. Watts is part of Columbia’s Collective Dynamics Group - an interdisciplinary team working on the "new science of networks". One of their insights are that non-homogenous networks (such as the internet) are more robust than homogenous ones (such as microsoft's integrated network of products), because there are no single crucial vulnerabilities that can bring the whole network down. Quote from the interview: "On September 12, 2001, one hundred thousand people had nowhere to go to work. But somehow, within a week, all those companies were functioning again—and they don’t even know how they did it. I attended a roundtable discussion with some of these people, and they said, well, we kind of did this and sort of did that and got some help from these people and more help from those people—and pretty soon we’re in an office somewhere. "
Bollen et al. (2002, 2003) have written about using network analysis to establish research trends in digital libraries.