One of the easiest sites is iLike.com ... First you need to register (super easy to do) and then select "Create a Playlist".
You'll title your playlist and then search for songs by title or artist. You can even search by generic terms related to your novel. Click on the arrow button to hear the song, and if you want to use it for your playlist, click on the plus sign.
After your playlist is complete, copy the URL or grab the embed code and add it to your site!
It took me less than five minutes to sign up and create this playlist for Shoreline (mostly based on songs I listened to while writing the manuscript!)
Have you used iLike? Are there other applications that work better?