Duet Display - With Duet Display, you can transform your iPhone or iPad into a second display to expand the screen space of your PC or Mac system.
scrcpy - Display and control your Android device from your computer
Astropad - Astropad is a free form drawing tool designed from the ground up to take advantages of the features offered by an iPad.
spacedesk - spacedesk expands the Windows desktop computer screen to other computers over the local area...
AirDroid - Access Android phone/tablet from computer remotely and securely. Manage SMS, files, photos and videos, WhatsApp, Line, WeChat and more on computer.
iDisplay - iDisplay is an app that turns a mobile device into a secondary desktop monitor. The app currently supports iOS, Android, Windows and MacOS, and it lets users connect a desktop computer to a mobile device over WiFi. Read more about iDisplay.