Deezer
The most avid Internauts probably already know about Deezer since it's been around for quite a few months. If you don't know about it, you should. Deezer has an impressive catalogue of songs that you can listen to free (and it's all legal, to boot!) from the Internet with no need to sign up.
The ever expanding Deezer database contains some of the most famous names in music as well as up-and-coming artists. Each track also has an embedded code (like the videos on YouTube), which means you can add it to your website or blog. You aren't required to sign up but if you do, you can take advantage of various extras, such as creating your own playlists, customizing a "radio station" and downloading and storing your own MP3s.
So with a fantastic resource, such as Deezer, who needs to download illegally anymore?
Manage your music
Organize your Mp3's
Lots of other free programs to manage and organize your music collection.