Go grab it here. I must say, I'm very happy with it, especially the music store integration.
I use Napster's subscription service. It lets me download all the music I want and listen to it all I want for $10/month. However, I loose access to all the music if I stop paying for the subscription. I love it because it lets me easily explore new music without having to commit after listening to a 30 second clip. It also lets me grab old albums that I listened to back in high school or college without having to shell out $10/album. I only wish they had more content, and I hate seeing "buy only" sections.
One of my biggest beefs with Napster was its client. The UI for managing music and selecting what to play is not very well thought out and I was always frustrated it. There were also little things that drove me crazy, like the media control buttons on my Dell Inspiron not working with Napster. You could play the music in WMP 9, but you had to jump through some hoops to get it into WMP's library and you had to manage your music in two places. With WMP 10, there are plug ins for the different music services. So, I can browse and download music from Napster directly within WMP 10 and it's very well integrated. I can now just use WMP 10's UI to organize and play my music, and that UI is quite nice in WMP 10.
A preview of the MSN music store is also available. The selection is still growing, so it's hard to judge its selection. At this point, there aren't any revolutionary features that I've seen. But, I love the fact that if I decide to purchase an album (either because it's not available through Napster's subscription or because I want to listen to it somewhere other than my computer) that I can go to any number of music stores in WMP 10 to find it. Choice is good :-)