I love finding anything that includes a combination of: simple + free + music. Mix Turtle, welcome to my life.
Here is a very simple idea that works very well… a search engine that looks for songs you want and lets you listen to them for free. Not sure how the mechanics work or if it’s even legal (not that I really care) but the simple interface, coupled with a little bit of AJAX love, provides a very nice user experience. Oh, and it’s pretty damn fast.