Air Display!

spacedesk is a windows network display monitor software that expands the Windows desktop computer screen to other computers over the local area network.

Duet makes your iPad and your Mac more powerful together. Slide Mac apps to your iPad as a second display. No lag, Duet helps you multi-task and work faster with more screen space. It doesn't require internet, just plug and play to get started and you can use the iPad's touch display too. Draw on the screen, produce beats with midi boards, design on the iPad and more.

AirPlay mirror your Mac or PC's screen to your Apple TV, wirelessly.

ZoneScreen is able to mirror local desktop to another computer in the network. The main difference is that it also allows extend desktop, not only mirror it. Also, you could use ZoneScreen for Windows Mobile to extend/mirror your desktop to screen of your handheld. All you need for ZoneScreen to work is network connection between your computers (your computers are both connected to the same network, or connected directly with cross-over cable or 1394 firewire cable, etc).

Smart devices become monitors by connecting them to a PC via USB! The Convenient Connect TwomonUSB

DisplayPad is a cutting edge app that allows you to use your iPhone as a second display for your Mac. You can extend your desktop with a wireless display that you can carry around with you!

Turn your iPad or iPhone into high performance extra monitor for your computer

AirMyPC allows you to wirelessly AirPlay mirror your Windows PC screen and sound to your TV through 2nd/3rd AppleTV. Basically AirMyPC lets you to mirror your computer display through your AppleTV just like you do with your iPhone and iPad. With AirMyPC installed - all these wires that connect your computer to your TV are not needed anymore. Once installed, AirMyPC streams whatever you see and hear on your PC to your Apple TV - wirelessly. With AirMyPC installed, with a few mouse clicks, you can easily stream your family videos and images from your Windows computer to your big TV screen »