This web page plays a song for as long as you want, but it doesn't just loop the song -- it analyzes the structure and loops segments that make sense next to each other.
labs.echonest.com/Uploader/index...
Pick an already-uploaded song just to see how they present the visualization of the song. For those of us who tend to listen to the same song over and over, this is pretty much the greatest thing in the world.
