what you listen to, learns what you like, gives you personal top charts and new music to discover. Last.fmI'm not sure what it means for the future - three plays for any track and you'll need to subscribe for future listening of that particular track.
As of [this week], you can play full-length tracks and entire albums for free on the Last.fm website. Last.fm
I played around with one of my favorite artists, Alison Kraus, and found her biography, all of her albums and most of her full tracks. One of the cool things is that like Amazon (and unlike Amazon you can listen to the FULL track, not just a snippet) Last.fm will suggest other like artists. This is a great way to discover more music. No one knows for sure how long the music will be free but take a listen yourself.