Ooh, looky here. MusicPlasma is big map that helps you find musical artists similar to the ones you already like. I’ve tested it out with several bands and it seems pretty reliable. Have a look.

Edit: Works really well with indie and techno. Not bad with classic rock. Acts that are less classifiable (i.e. not like many other acts) get the least reliable results. For instance, Joni Mitchell and Wesley Willis didn’t really lead to anything useful. But Radiohead opens up to a lot of options.