1,000 Recordings to Hear Before You Die by Tom Moon. The list is crazily diverse and very interesting. I find that I already have nearly all the pop and rock recordings (mostly albums) he suggests, as well as many of the classical pieces (though usually not by the artist he suggests). But even if I have the CD's and have already ripped them, I haven't necessarily paid close attention to all of these recordings before (at least not for a long time), so I'm finding gems in my own collection.
Where I'm weak is in jazz and in world music. In spite of what I call my "completeness obsession," I'm not taking the author literally, and I plan to take my time. I've enjoyed some world music (especially Indian), and I'm open to the possibility of being blown away by some African artist I've never heard of. And although I like a lot of "jazzy" pop and rock music, I've never had a great affinity for straight jazz. So I'm taking it slow, but I think this book will help me expand my musical horizons in several directions.