Napster now offers free on-demand streaming of many of the songs in their catalog. Although it isn't as high quality as their paid service, it is easier to use (no software to install), meaning you can stream music anywhere you have a connected pc (like work, your friend's party, etc). I am still a big fan (and paying customer) of Yahoo! Music Engine / Yahoo! Music Unlimited, but this streaming service is ondemand so it beats the Launch.com streams I usually resort to when I need streaming music away from home.
Of course, the best streaming music station, in my opinion, is WOXY - BAM! The Future of Rock 'n Roll