I've been listening to a lot of The Magnetic Fields recently and decided to see what sort of data I could collect from last.fm about similar artists. I started on the similarity page for The Magnetic Fields, and wrote down the first two artists. Then I went to each of those artists similarity pages and did the same thing, producing a tree of artists that are similar to one another. The tree of highest similarity led from The Magnetic Fields to Belle & Sebastian to The Arcade Fire, and finally to Bloc Party, which led back to The Arcade Fire. Including the second most similar artists yielded a total of thirteen artists before all the links led to artists I already listed.
Here's what the similarity diagram looks like (click on the diagram for a larger version).
Artists in black are those that were directly connected to The Magnetic Fields or related artists during my initial search, the dark blue arrows are links between the most similar artists and the lighter blue arrows are the number two links. The numbers near the artist boxes are the number of artists that point to them.
After I made the original diagram, I wanted to see where some of my other artists that I thought might fit into the diagram. I added Of Montreal, Architecture in Helsinki, The New Pornographers, and Spoon, again, looking for the top two most popular artists for the new groups. I was certainly right that these artists are similar, although none of them were directly connected to The Magnetic Fields. Of Montreal is only two links away (Of Montreal and The Magnetic Fields both point to Neutral Milk Hotel), and the others I added are three links away.
I think it would be really interesting to figure out some way to generate these sorts of diagrams automatically (since the connections will change as the listening habits of last.fm submitters change), and to add more levels. I used Metapost to build the diagram, so the diagrams could be built programmatically, but it would take some work to read and follow the appropriate links from the last.fm web site.