Duet lets you use your iPad or iPhone as an external monitor.

Mirroring360 is a collaboration tool suite for your to wirelessly mirror, share and record your Chromebook, iPad, iPhone, Android, Windows and Mac screens.

ApowerMirror is a wireless screen mirroring app that allows streaming of your Android or iOS device to any Mac or Windows computer.

Air Display turns your iOS screen into a wireless (or USB) display for your Mac.