Greg says: Pandora is the best music-related technology I've run into in a long time. Simply put, it's custom streaming radio based on your musical tastes. Doesn't sound that revolutionary, but the "Music Genome" project underneath it does a really good job of coming up with new music that fits your tastes, based on seed artists and songs you provide it.
I couldn't agree more, Pandora is really impressive! It just took me a few "I like" and "I don't like" to get quite consistently music that I like (including lots of things I didn't know and would never have listened to otherwise). I'm hooked.