No April Fools here. You really can use your iPad as a second PC monitor.
Using two monitors really enhances productivity. You can read something on one screen while you type on the other. Or, you can copy something on one screen and paste them to an application running on the second screen. At work, I use two monitors but when I'm traveling, I'm often confined to a single monitor which is my laptop screen. There are apps that can transform my iPad (or iPad mini) to act as a second monitor.
Air Display by Avatron is an example of one of these apps that lets you use a spare mobile device as a second monitor for your Mac or PC. You have the option to use a spare Mac, PC, iPad, or Android tablet to serve as a second monitor. In other words, you can configure your second monitor like this:
Mac -> Mac
Mac -> Windows
Mac -> iPad
Mac -> Android
Windows -> Mac
Windows -> Windows
Windows -> iPad
Windows -> Android
You need to purchase separate apps for each second monitor configuration. I chose to use my iOS devices as a second monitor, so I grabbed my iPad 3 and my iPad mini.
There is a little lag when you move windows around on the second monitor, but overall the app works quite well. I spent most of my time using my iPad 3 as my second monitor. My iPad mini worked as well, but it definitely felt slower than the iPad 3. My primary PC was a Windows 7 laptop.
All you need is the Air Display app (running on your second monitor device), drivers for your primary device, and your devices need to be connected on the same Wi-Fi network.