I remember when I was 14 or 15, I was going to our small-town's public library once every 2 days or so, I was reading all the SF books I could find. The thing is that the SF books shelf was just after the history books shelf, and, even though I liked history, I never would have thought of reading history books if it weren't for me passing in front of that history books shelf each and every time.
Google could handle this with "I'm Feeling Lucky." I seem to remember delicious.com doing something like this. How about a serendipity web app that tries to figure out/crowdsource what you never knew you'd be interested in?