Yesterday (Tuesday May 10th, 2011) Google released its music locker. It's currently invite only and you can request an invite at http://music.google.com/about/. Its main purpose is to keep all of your music in sync across all of your devices however, it's currently limited to only desktop and android operating systems.
The Android app:
Android users can access their music via the android app available in the market. Using the app doesn't take up any extra storage space on your device because all of the songs are streamed however, if you suddenly lose connection your music will be cached onto your device for offline listening. Also while using the app you can select certain artists or albums that you'd like to have downloaded to your phone. Another interesting feature is the instant mix option that is similar to iTunes Genius or the popular internet radio station Pandora, what it does is selects songs that are similar to the song you've selected. Last thing I'd like to mention is that you'll be able to manage your playlists on the go and your changes will be updated with Google and you'll be able to access your changes on any computer.