Amazon Prime has to be one of my favorite services. You basically get a subscription of Netflix, shipping, and now music for under $100 a year!
Amazon Music was only released a few days ago, and I'm already pumped. You can now look through hundreds of albums, "add them to MP3 library" and download them to your computer's or tablet's library. In your library, there are no restrictions or ads to play the Prime music. (After your Prime membership expires, your music will disappear.)
Some of the great albums I personally would suggest from the list here
Personally, I was already a fan of Prime for the movie and television selection, but this is a really cool benefit! What are you going to be adding to your library?