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.
Ask Different is a question and answer site for power users of Apple hardware and software. It only takes a minute to sign up.Sign up to join this community
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
The little square with the triangle at the bottom is Part of Crome Cast.