1. Pandora: Internet Radio
"Pandora is a music discovery service designed to help you enjoy music you already know, and to help you discover new music you'll love." You choose an artist or song and Pandora will create a "station" based on your song choice. Pandora will suggest new music to you, you can play and then rate that music. Over time Pandora gets to know your musical taste and can introduce you to new artists as well as play old favorites. There is even an iPhone App that will allow you to stream music on your phone.
2. Just Hear It: Search the web for any song in the world.
Just Hear It is an on-demand music discovery service. You can search for and listen to any song in the world without signing up or paying anything. You can create your own playlists and legally stream your songs right from your desktop. They pay for licenses from all the major performing-rights organizations. Just Hear It is currently in private beta, you can search and play music without an account. Members (by invitation only) can also create a media library, multiple playlists and keep track of their play history.