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.
What a great concept! And it looks really good too. I'm not sure how I'd use this, since I'm not conducting research at this level, but..
There are a lot of user-built maps which are great to look at.. Check out linkages for the CFR (Council Of Foreign Relations).. Lot's of mates there!