Top cover songs
I've always been a big fan of cover songs. They provide a great way to experience old music in a new way. They can help you discover a new artist or a new genre - by combining the familiar with the novel. To build the ultimate cover song playlist I used the Smart Playlist Builder to create a playlist of covers that most frequently appear in cover songs playlists. These are the essential covers. Have a listen:
http://www.rdio.com/people/plamere/playlists/5775766/Top_covers_songs_via_SPB/
You can read more about the Smart Playlist Builder and create your own wisdom-of-the-crowds playlists.