Enables controlling audio / video media (i.e. Youtube, Spotify, SoundCloud and more) across all tabs with a single button
It can play, pause, go to next or previous songs on all supported sites.
Comes in handy when someone sends you, say, a Youtube video link and you happen to be listening to Spotify at the moment - just hit the appropriate button to pause Spotify without need to switch through the tabs. When you are done with that funny YT cats compilation video, you can simply resume your music :)
It is also open source: