What app is this icon with a square and a black triangle? I've seen this icon in a tutorial I watched on YouTube. What app is this icon?
Have searched through Google Images but found nothing.
That is the icon for AirPlay, by Apple, and that menu bar item allows you to control the streaming/mirroring of content on your Mac to an Apple TV or other AirPlay server.


  Turning AirPlay Mirroring on and off
  
  If your Mac supports using your Apple TV as a display, an AirPlay menu appears in the menu bar when an Apple TV is on the same network. The AirPlay Mirroring menu icon turns blue when AirPlay Mirroring is on ().
  
  


Source: About AirPlay Mirroring in OS X