Spotify.com

Spotify offers free and legal access to a huge library of music. The free music option includes adverts, but there are also two paid-for options which don't have adverts. The one is a 'day-pass', and the other a 'monthly subscription' which allows you to download tracks in MP3 format so that they can be played even when you aren't connected to the internet.

To use the service you need to create an account and download the streaming music player. The Spotify music player is similar to iTunes, and similarly you are able to buy and download tracks.