They Rule

This site was mentioned on the News Library List:
They Rule aims to provide a glimpse of some of the relationships of the US ruling class. It takes as its focus the boards of some of the most powerful U.S. companies, which share many of the same directors. Some individuals sit on 5, 6 or 7 of the top 500 companies. It allows users to browse through these interlocking directories and run searches on the boards and companies. A user can save a map of connections complete with their annotations and email links to these maps to others. They Rule is a starting point for research about these powerful individuals and corporations.

They Rule allows you to create maps of the interlocking directories of the top companies in the US in 2004. The data was collected from their websites and SEC filings in early 2004, so it may not be completely accurate - companies merge and disappear and directors shift boards.