Apple - What app is this icon with a square and a black triangle?

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}