If you like listening to and learning new music then you'll enjoy what Pandora.com is offering
is so easy to use and enjoy that it isn't necessary to get into the details of how it works but I think a quick overview will give you the gist of what it's about.
The only thing you need to know is that it offers a lot of good quality music which is freely available to listen to and it learns what you like by the choices you make.
You can set up different "radio stations" depending on what you enjoy listening to and the stations can constantly be tweaked by adding music or artists.
Initially when you're trying to refine your stations you may find yourself skipping a lot of songs. Unfortunately the license agreement they have only permits so many skips per hour so sometimes your stuck listening to songs you don't want to.
There is no standalone player which may not be a bad thing for some people but depending on what you're using your computer for, another open browser window may be annoying.
Just Try It
Enough talk, just head over to Pandora.com
and give it a try. It's so easy to use and the site does a great job of explaining everything.