As if there weren't enough contenders out there for music subscription service, Slacker Radio has updated its business model. You can still get the standard ad-supported radio station features that were always available. However the company has added a Spotify-like buffet option for $10/month. Pony up the dough and you can remove all ads and listen to as many tracks as you want.
The combination is pretty powerful, as one of the biggest complaints over Spotify is its discovery problems. Slacker Radio never really dethroned Pandora, but it did offer some stiff competition. There has been relatively little overlap between radio and subscription services.