Pandora
I know this is probably old hat, but I am quite impressed by Pandora. The general idea is that music is classified by various characteristics, by hand. You tell it what you like and what you don’t like, and it selects tracks it thinks will suit you. Unlike the usual social networking style of recommendation, this approach appears to work.
Most systems I’ve tried consistently recommend things I hate (I have a suspicion that this is because most of them don’t actually let me say “I hate this”, though – last.fm being a good example of this), mingled in with stuff that’s OK but not earth-shaking. In sharp contrast, Pandora supplied me with a bunch of pretty good music, much of it by people I’ve never heard of, after almost no training at all.