The music playback rental.
Play full CDs on your PC, as many as you want. If you have the right portable, download as many tracks as you want, but they're DRM protected to expire.
I have Napster to try CDs before I buy them from Amazon, and to listen to things I don't like enough to buy real copies of.
It's pretty handy when someone says "Bruce Cockburn is great!" to be able to legally hear all of Breakfast in New Orleans or You've Never Seen Everything, without buying a copy of either.