iRATE uses open source technology and uses freely available music, so not only will it not cost you anything to set up, but it won't run you afoul of the RIAA when you use it. Sounds like a winner to me.
The way it works: you download the player, and then it downloads some random tracks. You listen, you rate (hence the "iRATE" in the name) and then based on your ratings, it throws some more tracks in your playlist. The more you rate, the more you get, and the more accurate (theoretically) its suggestions become. It seems like a pretty cool idea, and it'll provide a few days of entertainment at least. It may even end up providing more than that, but a lot depends on the quality of the music it has access to.
Download the client here (Windows, Linux, OS X, and source code...):
(Yes, I know it says "unstable", but the "stable" version is so feature-free that it's well worth moving to the unstable branch. If you use the Java Webstart version, it'll automatically download it and install it and run it for you if you let it, without any effort, assuming you've got a recent version of Java installed. After running it, if it works for you, I advise closing the app and clicking to it again from the download page - if it detects that it's your second time using it, it'll install shortcuts for you if you want, making it much easier to start later.)