iRATE radio is nearing its 0.2.1 release. Many problems have been fixed, so it's expected this version will dramatically increase its popularity. But your help is needed for the final testing to discover any remaining problems.
iRATE radio is a collaborative filtering client/server mp3 player/downloader. The iRATE server has a large database of music. You rate the tracks and it uses your ratings and other peoples to guess what you'll like. The tracks are downloaded from websites which allow free and legal downloads of their music.
iRATE radio is a Java program. It is available compiled native for Windows or Linux with GCJ, or as OS-independent bytecodes for any platform as a Java WebStart download.
I'm really impressed with what Sun has accomplished with WebStart. You really should try it out just to see how WebStart installation works. We could all learn a lot from that. However, WebStart on Linux doesn't work by just clicking a link. You have to download a small file, then give it as a command-line argument to the "javaws" program. On Mac and Windows, you just click a link and your program gets installed. It even checks for a digital signature on the binary.
For Mac OS X you will want the WebStart download. It is trivial to install; just click the link and in less than a minute you'll be running iRATE. For Windows, use the GCJ version, or if you'd like to test WebStart on Windows, you'll need to install the Java Runtime Environment 1.4.