Thanks to Carrie, I've discovered Pandora - a fun way to discover new music. It tries to learn what sorts of music you like based on artists and songs you add your "radio station". You can then vote for similar songs that Pandora picks, and it will include your feedback in future music.
It reminds me a bit of MusicPlasma (now LivePlasma), except that Pandora streams the music to you for free, instead of just showing you related bands and letting you browse for albums. Both have slick UI designs, with Pandora's being a nice alternative to the standard web page/tab sheets/popups design - very appealing and easy to use. Vista's new Start menu uses a similar approach, but is not as fluid.
If you want to listen to some of the stuff I like, here's a station I've built. (Click to listen in your browser)