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.
Duet offers one of the best experiences for temporarily extending your desktop setup when you're on the go. With Windows support now available, Duet Display can now be used to extend the desktop of just about every computer on the planet. We've been using Duet for Windows for a couple of days now; while it's not yet perfect, the experience is thoroughly enjoyable.
Much like the OS X client, Duet Display's Windows app is designed to be a plug and play experience. Install the Windows version and open the app on the iPad; as soon as you connect your iPad's Lightning cable to your PC, Windows treats the device like a second monitor. After a brief screen refresh, you can drag applications from one desktop to another — exactly as though you had a monitor connected via HDMI. Input to the iPad's multitouch screen outputs as mouse movement: It means you don't get swipe gestures like you would on a native touch device, but you can still tap and double-tap to open and adjust things.
It's early days for Windows support, however, and there are a few setups that don't work quite right: In our testing, SLI configurations aren't fully supported; installing Duet Display on these setups causes immediate problems on reboot, but uninstalling the app will quickly eliminate those issues. In talking with the developer, though, they plan to resolve those problems quickly. Every other PC environment we had available and tested handled pairing with Duet Display just fine, with the occasional stutter in the Windows 8 full screen start menu.
This is by far the most elegant portable multi-monitor display out there today, especially if you're already using an iPad and a laptop on a regular basis. Duet Display is available today in the App Store for $15.99, with the Windows and Mac clients available on the Duet website.