
I've just started using mflow which seems to be a pretty good (free, ad-free) alternative to Spotify, but I know it is only a matter of time before it goes the way of all those who came before and erects paywalls all over the place.
I've always thought that it might be a good idea for Amazon to do something like this. Major clout with content providers so it has a better chance of negotiating a good price, an existing MP3 store that they can link in to, the IT infrastructure to deal with the load, Amazon clouddrive tie in for those who want their purchases/music collections available anywhere.
The trade off: Amazon would also get to track what you listen to so that it can better target its advertising.