While Spotify has been a major force in digital music since its introduction, you needed to pay $9.99 a month to access the service on mobile devices such as tablets and smart phones. Until today, that is. Now Spotify is offering free, ad-sponsored music on all supported devices.

This is a big change, and it will put a lot of pressure on Spotify's competitors, which are as varied as Pandora, Rhapsody, Nokia (MixRadio), Apple (iTunes Radio) and Microsoft (Xbox Music Pass). That said, there are some curious limitations.

Previous to this change, Spotify offered three levels of service: A free, ad-based version that worked only on PCs, a $4.99 per month Spotify Unlimited option that removed the ads, and the $9.99 per month Spotify Premium, which added mobile device and offline playback support.

Now there are only two options. These are:

Spotify. The free service works on smart phones, tablets, and PCs, and is ad-supported. But playback on smart phones (iPhone and Android, the site says) is limited to shuffle play, so you can't arbitrarily just play any song you like. (You can however shuffle play whatever playlists you've created.) On iPad and Android tablets, oddly, it works like a PC: you can play any song or album, on-demand.

Spotify Premium. For $9.99 per month, you can play any song, anytime, on any device, without ads. You can download music and listen to it offline.

Since there's a Spotify app for Windows Phone, I'm not clear why the news release only mentions iOS and Android; perhaps the Windows Phone app needs to be updated to understand what's possible with the new service levels.

But regardless, I think Xbox Music is a better deal, especially for those that use multiple device types. Xbox Music works on Windows 7, Windows 8/RT, Windows 8.1/RT 8.1, Windows Phone 7.x, Windows Phone 8, the web, and on iOS (iPhone, iPod touch) and Android (handsets and mini-tablets) and has improved dramatically over the past year. With an Xbox Music Pass, you get everything you get with Spotify Premium, and you can even save a lot of money by paying for the annual subscription. And there's no weird shuffle-only limitation on handsets.