Media Keys is a small WebSocket server that run on OS X, emitting events when you press the "media keys" on your keyboard (F7, F8, and F9). With this you can, amongst other things, control web-based music streaming services. In the extensions folder, you will find Safari and Chrome extensions that control playback on Rdio.
Credit for the idea of using WebSockets: Boris Smus
Note: the server starts with the system and is always running. It will always be consuming the media key events whole and no other applications (e.g. iTunes, QuickTime Player, etc) will recieve them.
Building from source
If not already installed, this project will install libwebsockets.
$ git clone --recursive git://github.com/whymarrh/media-keys.git $ ./install.sh
Add the Chrome extension found in the extensions folder (the
Media Keys.crx file) to Google Chrome (via drag'n'drop).