If you've not heard of it...

www.pandora.com Totally free, no malware, bad popups, etc.

You feed it artists and songs that you like and then it custom/adaptively stream music that is similar to what you gave it. Then you can, if you want, say if you like or dislike a song. You can also bookmark songs to keep track of new things you like.

Basically, just has a little ad floating on top of the player, that's it.

I highly recommend it if you're interested in something a little different. It's pretty good in that you can essentially have a free streaming jukebox of stuff YOU like.