Air Display: iPhone, iPad, Android, Mac or Windows PC as an external monitor. I love this app. I already use AirDroid to show the screen of my smartphone on a computer or project it on a big screen. Now, using Air Display, I can extend my Windows desktop to my Android tablet, and use the tablet as a second monitor. You have to be on the same WiFi network; but, this little $9.95 app is well worth the cost. I have a USB second monitor that I sometimes use to edit documents. But, my 10.1 inch Samsung tablet is small enough, but big enough, to sit next to my big screen at work and at home, and do productive work. Air Display also works with iPad and Mac, and was designed for them; but, now that the company has recognized the huge Android and Windows market, I predict big sales gains.
Note: After posting, I discovered that AirDisplay doesn't yet support Windows 8. So, I will have to use it with a backup computer.