With all the fear, paranoia and general silliness about Goldman Sachs these days, this figure from a new Science article is worth a look. It uses transaction relationships to map the financial sector, showings its connections and centers.
Here is the description:
The identification in the nodes represent major financial institutions and the links are both directed and weighted and represent the strongest existing relations among them. Node colors express different geographical areas: European Union members (red), North America (blue), other countries (green). Even with the reduced number of links displayed in the figure, relative to the true world economy, the network shows a high connectivity among the financial institutions that have mutual share-holdings and closed loops involving several nodes.
Frank Schweitzer et al., “Economic Networks: The New Challenges,” Science 325, no. 5939 (July 24, 2009): 422-425.