Now that buying music is a la carte, collections tend to be made of singles; while you obviously own "U Can't Touch This", "Tell Me Have You Seen Her?" has to pray just to make it today. Now finding the deep cuts for you, I Like B-Sides.
From a Boston-based web developer, Sides uses a "mathematical proprietary formula" to examine your iTunes library and suggest "random mixes of less popular but enjoyable songs by the artists you already like", but will soon hate if you can't find a second song by them to listen to. It works like this: just compress and load your iTunes library, and Sides'll see what artists you play all the time, then check sales data to find obscure, underplayed songs you don't own from said artists -- 16 fresh tracks calibrated carefully to ensure "a proper ratio of quality-vs.-quantity", eerily mirroring your college dating habits. Once you get their selections, listen to 30-second previews, download immediately using Amazon MP3, or, if you're not feeling those particular picks, just hit "Shuffle My B-Side!!" to get a completely new mix-tape -- an option sadly unavailable with stepdads.
To find more gold in your existing mine, check a bar graph of your iTunes listening habits and suggestions for "cool tracks you have + are probably ignoring", a service that'll make you say "Oh, my Lord. Thank you for blessing me."