Music+ is a plugin that was developed during the Echo Nest Social Music App Challenge. It uses the music intelligence service's API (as well as Facebook's API) to make it easy to find and listen to music while surfing the web.
Simply install the plugin in Chrome, and start discovering music. At its simplest, you can pull up a toolbar in which you can search for playable music to listen to while browsing (courtesy of Rdio and MP3s on blogs). Those songs can be shared with friends on Facebook via message, or they can be posted as a playable stream on your wall.
Start surfing, and the app gets interesting. If you go to a blog and highlight a band name and right-click, you can listen to songs by that artist instantly. However, the app really shines when used with Facebook. You'll have to disable secure browsing in order for it to work (which is a pain), but once you do, every artist on the site will have a "Play" button next to their name, allowing you to listen to tracks by that artist right within Facebook. You can then surf around the site as usual, without having to stay on the page to continue listening (as seen with Facebook Page apps like Bandpage).
The app also adds a column to the left of any band Page containing similar artists, an awesome addition when it comes to music discovery. It also allows you to add artists to your queue for later listening.