I don’t know how to accurately describe this, so I’ll mangle it instead. Liveplasma lets you search for a band you like (say, Wilco) and then groups other bands you may like by proximity. The UI is a little wonky (horizontal scrolls bars are bad, UI people, bad, especially if you’re using Flash), but the feedback is interesting. I already added one album to iTunes because of it. (Hint, hint)
Check it out, it’s a very cool idea. I keep imagining what a tool like this could do if it were tied to, say, my Amazon account or past searches. (Welcome back, Ryan! Since you last searched for Goo Goo Dolls, we’ve associated x number of bands with them. Check it out!)
Oh, and it works for books and movies, too.