I second Last.FM.
It's also a place where you can you listen to Kym's awesome music unlimited times for free.
There are also songs on Last.FM that you can listen for free up to 3 times like Underground by David Bowie (before you have subscribe, buy the cd, download the song on iTunes or get lucky on the Last.FM software's search if you want to listen to the song again) while others like Here Without You by 3 Doors Down can be listened to unlimited for free.