The folks behind Duet Display have released a Windows client, which allows you to extend your PC to any iOS device.
Plugging in a second monitor to extend your computer's display is about as far from a new concept as you can get, but taking an existing piece of technology and using it as a second monitor is something of a challenge. Duet Display was one of the first reliable apps to turn your iOS device into a 60fps second monitor with touch input; now, the app has added support for extending Windows desktops.
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