As Elizabeth said, Pandora is one of the best ways to find new music and if you take the time to 'train' the software by giving songs thumbs-up or thumbs-down and using the 'more like this' function you can fine tune a station to your liking.
The premium subscription, Pandora One at $36 dollars a year, is definitely worthwhile. There's no advertising, you can skip as many songs as you like and the music stream is higher quality than the free version.