The site is called 'I like b-sides', and doesn't require any registration or sign-in. Off to a good start. What you do is upload the “iTunes Music Library” text file (normally in My Music/iTunes), and the site offers up a number of songs not on your playlist that you might also like. It'll even let you sample them from Amazon, with the obvious choice to buy (I guess they get a cut). Don't like the choice; there's a shuffle button. Want even more? There's a list of songs off your upload that you might be ignoring. I'm not sure what criteria they use for this; maybe an average rating from all the uploads.
My only disappointment was that the results didn't offer any new artists, but I'm not sure if that would be true for everyone.
To discover new artists, I use 2 sources:
Amazon.com or Amazon.co.uk - simply search for an album you like, and then look at the "Customers Who Bought This Item Also Bought" section. With Amazon's turnover, you're bound to find something you like.
Last.fm - if you enter a songs name, they'll offer up a playlist of similar music. Even better, if you're prepared to create an account, and 'scrobble' your music for a while, they'll offer up quite meaningful alternative bands and albums. Last.fm has a lot of another useful and interesting features, like adding friends, and seeing how compatible your taste in music is to others, because it has a strong social side to it. If you want to give it a try, look me up. My username is Brat7.
Thanks for reading.