Discovr is a cool little app that will help you find new artists related to the ones you already like. Not only will it show you an endless bubble chart of artists, but it will also give you; music samples, biographies, links, and an awesome interface. You just search one artist you like to get started, more will appear. Check those artists out, if you like one of them, tap on them, and find artists related to that artist. =]
Discovr is $2.99 and only for the iPad. Be sure to check it out in the App Store.
Get it here:
Discovr for iPad on the iTunes App Store