FreePlay has 15,000 songs and is growing every day. They have a neat homepage with an efficient searchbox. They are at the forefront of innovations in the music industry, and have an agreement with YouTube to offer free music. All of the tracks are downloadable absolutely free if you are just fooling around and this stuff is non-profit/non-commercial. If it is, however you need to go about purchasing licensing.
Pricing: about 40$ for a royalty free music