Grooveshark is a great music streaming service but it's stuck in your browser unless you pay for the premium service that allows you to download and use the desktop app. GrooveApp gives you a menu applet the allows you to control Grooveshark with your media buttons as well as a window which it loads the player into. So you basically have the full Grooveshark experience for free.
I listen to Grooveshark all the time and my biggest complaint is that when I want to listen to something other than Grooveshark I have to find the window and pause the music. I can't seems to find a Firefox browser plugin or controller that works at least not in the long run. GrooveApp solves both these problems by removing the player from the browser and giving you control over the playback using the media buttons. I've written about Factotum
which gives you media key access to many online music service, but it only works for Safari and Chrome. I'm a Firefox kind of guy so that doesn't solve my problem either.
GrooveApp is still in it's youth as far as development goes, but it seems to work pretty well. It has Growl support and does give you ability to use the media button which are very responsive in my experience. If you are a Mac user and love Grooveshark give GrooveApp a try.