Slacker Radio is like last.fm - it builds personalized radio stations. You get to rate songs and artists as "like" and "hate" to make them appear more frequently or to ban them from your radio station.
Online service available as a device that downloads and caches radio stations using a PC like a cross between an ipod and last.fm.
Is also a free app for Blackberry and iPod Touch and iPhone.
Finally, you can just stream direct from the website too.
All are synced up. Mobile app will download a disconnected cache for your stations, too. The "matching" libraies are well thought out.
