I like b-sides is a new service from the maker of the now-defunct DiggSuggest. The idea is simple: you go to the Web site and upload the metadata from your iTunes library (in an XML file), and the site tells you songs that you might like, based on the bands it knows you do.
I tried this out with a small library of about 4GB and it gave me a good sampling of tracks. All of them come from Amazon's MP3 store, and can be previewed in a small player that sits on the side. There are also links that will take you to each song's download page, giving i like b-sides affiliate credit in the process.
The tool's secret sauce is in a small feature off to the side that calls out "cool tracks you have and are probably ignoring." These are tracks deemed popular that simply have low play counts. This same section also tells you how much of your library has independent artists, and how many songs you've bothered to rate. I'd love to see both of these go a little deeper.
While there's no built-in social element with friends or following, you can share the tool's recommendations with others. Each library gets a permalink to do this, and you or your friends can come back and "shuffle" to bring in new results (you can try this here). The bigger library you have, the more diverse of a selection you'll get, so revisiting the site as you acquire new music will get you even more recommendations.