Although we're fans of Zune (or LastFM) on our Windows Phone, on occasion we get the request for a Google Music (music.google.com/) client. While we're not holding our breath for an official app from Google anytime soon, there's no reason why devs can't do it on their own.
For those who don't know, Google allows you to upload your entire music library to the cloud and gives you the ability to stream anywhere. It's not actually a bad idea as it allows you to backup your collection, free up hard disk space and easily download it again when needed. The service is free for your first 20,000 songs which should get your started (though uploading 20K songs may take some time). [Side note: We're really hoping Microsoft takes SkyDrive + Zune in this direction for Windows Phone 8 and the desktop OS].
Today, Gooroovster is now available on the Windows Phone Marketplace. The app goes for $3.99 which is on the high side but there is of course a free trial. The app has been in private beta for weeks though and initial feedback for the app has been very high, making this seemingly a great solution for those with Google Music as their primary music resource. (Beta testers will be getting one final update very soon to match the released version--kudos to the dev for that).
Allowing you stream via the Zune music player under lockscreen/background and access your entire library, the app is exactly what many have asked for as it even has a very Metro UI. For that, we're giving it our seal of approval.
Give Gooroovster a shot here in the Marketplace and submit feedback and requests via the UserVoice forum.