Mountie - Mount a second screen (iPad/iPhone/tablet) to your MacBook
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.
shiftscreen for iPad and iPhone - External display for your iPad/iPhone!
Mountie+ - Connect a second (or third) screen to your laptop
Luna Display - Hardware that turns the iPad into a wireless second display
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.