Music Bubbles is an awesome controller for Google Music.
- the controller is hidden if the player is is not runing
- the icon is simple and at the right alpha channel so that it does not overtake any webpage that it's displayed in
- there is an animated progress bar which is also subtle
- the controls are reasonable
- you can "blacklist" a particular address so that the control does not interfere with the page
My only complaint would be that on a ChromeOS installation it would be nicer if it were a desktop icon instead of a browser control. Of course this is only a minor complaint.