Google Music was introduced on November 16th, 2011. Google Music features Google Play and Google+ integration. Google Music is a cloud media player that supports streaming to desktop browsers and Android devices (or any device that can run Adobe Flash). The service is great because you can upload your own music to access later from the cloud. There is a 20,000 song limit. Google Music has a feature called Instant Mix that creates a playlist of songs that go well together. The interface is slick and you can purchase songs through their music store.
Google Music on blackberry: http://crackberry.com/make-use-google-music-your-blackberry-playbook-using-googmusic
Google music on iphone: http://www.mobileburn.com/18970/review/getting-google-music-on-your-iphone–a-google-music-ios-app-roundup
Google music on windows phone: http://www.wpcentral.com/third-party-google-music-client-windows-phone-now-available