This is a very cool application, Pandora. Its brought to you by the folks at the Music Genome Project. Pandora allows you to create your own customized music stations perse. You start with a blank slate, then add an artist or genre, etc and then Pandora will start streaming tunes based on the data you feed it. Whats cool, is that you can then continue to add additional artists and genres to your station and the system will continue learning your interests. Pandora keeps on playing music and exposing you to new tunage that you may have never heard before (thats the beauty of it). You can create multiple stations, launch the player and keep it on your desktop etc..
Of course, using the player and creating is completely free, you just have to register after a certain amount of use. In order to keep the service free, it would appear that the Music Genome Project has some kind of advertising arrangement with Apple, since iTunes ads rotate continuously. You can of course also purchase any tune you are listening to via iTunes directly from the player, or also purchase the album via Amazon.com. I think its a great idea. Check it out, build a station.