Goombah looks at the contents of your iTunes library, and generates a list of similar music, and a list of other people who use Goombah with similar tastes. From these lists, you have the ability do purchase the tracks from iTunes.
Goobah also displays "Goobah's Free Music". This this is a collection of free music. Goombah uses bittorrent for the downloads, so they should be pretty fast.
Unfortunately, I wish there was the ability to generate a playlist of the tracks already in iTunes, that are the most popular, and have those playlists to added to iTunes.