This past summer I worked with the Digital Library Research and Prototyping Team at LANL. One of the projects they were working on, a "map of science", was just featured on the Wired Science Blog. The graph below is based on a massive collection of scholarly usage data (click on the image to get a more detailed look at it). Basically, the graph shows how users accessing scholarly work in one field (e.g., Cognitive Science) may also access work in another related field (e.g., Sports Medicine).
You can find more information about this work in:
Clickstream Data Yields High-Resolution Maps of Science by Johan Bollen, Herbert Van de Sompel, Aric Hagberg, Luis Bettencourt, Ryan Chute, Marko A. Rodriguez, Lyudmila Balakireva. Public Library of Science ONE, March 11, 2009.