This is a really interesting visualization from Spatial Variation in Search Engine Queries, a WWW08 paper by Backstrom & Kleinberg (Cornell) and Kumar & Novak (Yahoo!).
What you are seeing are the "spheres of influence" for various Major League baseball teams. To create this graph, the authors mined the search engine logs from Yahoo! and plotted where the most popular baseball team queries originated from (based on IP address).
The result is a geographical break-down of each team's fan base. The authors point out that the boundaries even follow state boundaries closely: "For instance, in Michigan, across the lake from Chicago but far from Detroit, it is the Tigers, not the Cubs who have the largest following."
It's also interesting to note there are some places like Arkansas where there is no single winner. That's what happens when you live in a state with zero professional teams. Oh, how I miss my childhood years living in Denver...