As mentioned in my LEARNING TO IMPROVISE - LISTENING article, I've used all of the major online music services (eMusic, Napster, Rhapsody, Yahoo, and iTunes). Like Rhapsody, Napster and Yahoo let you listen to an unlimited number of full tracks as part of your subscription. Rhapsody has been my favorite, though, due to its great jazz selection and streaming audio quality.
Rhapsody recently released a new version of their software that improves their user-interface in both features and functionality (although there are still some glitches). They also just released a Web-based version that can be used on just about any operating system. It isn't as nice as the software version, but it's obviously more accessible and it doesn't require an install.
My favorite thing about the Web-based version is the fact that anyone (even non-members) can go there and see exactly what they have to offer. Just enter your search terms in the box at the top, and click on a matching artist name. As a member, you can listen to all of those albums for just $10 a month!
Try a search for yourself at http://www.rhapsody.com/
Why am I promoting this service? Aspiring jazz musicians need to listen to a lot of jazz. With Rhapsody, you'll have thousands of great jazz recordings to learn from and you won't go broke in the process. In fact, a service like Rhapsody frees up more money for you to see more live jazz!