I was introduced to Pandora today, the free web radio. I'm certain there's been something similar to this before, but I have to admit, this one works pretty well.
The basic concept is easy to understand: you make a "station" by choosing a song or artist you like. You then listen to that song/artist, and rank it (like/dislike). Pandora will compare that information to their Music Genome Project, their six year project where they broke thousands of songs into individual music qualities. Then, you get an unending series of songs that will, theoretically, be in the same vein you like. If you like a song, rate it up. If you don't, rate it down.
Best of all, you can have more than one "station". Currently, I have one primed with Tom Waits and one primed with White Stripes/Franz Ferdinand. I'll shortly add a third, this time using Monster Magnet as a base. Definitely not stuff I want to get mixed up :)
The interface could use a little loving, though. Once someone helped walk me through it, it was very straight forward. Lots of useful help text, letting me know what it was doing every step of the way. But if I had stumble through it the first time, I would probably have given up in frustration. The interface is designed to be compact, but suffers from "what am I supposed to do now" -itis. Also, their song licensing is a bit limiting, as you cannot rewind or go back to songs, nor can you skip ahead more than a handfuil of songs an hour. If you think of it as a radio rather than a CD player, it makes more sense.
If you hit the site for the first time, I recommend reading the FAQ and the About page, both links annoyingly tucked away at the bottom of the homepage. Oh, and it only accepts USA postal codes, so us canucks have to be creative :)