MaxiVista

MaxiVista allows you to add a second screen to your computer. By installing this app into your primary computer and a spare computer, you can use the spare computer's monitor as a second screen for your main computer.

Both computers have to run a version of Windows and should be connected to the same network. When you do that, you can activate the second screen from your primary computer via the MaxiVista tray icon.

Also, you can remote control the second computer from the primary one.
From the primary computer, you can change the screen properties of the second screen. You can set the display to a portrait or a landscape and even play with its resolution. You can also place the second monitor to the right or left of your primary computer, or even above or below.

MaxiVista gives you some options to control the size of the secondary display as well. If the second computer you are installing MaxiVista into is too small, there are options to have the content fit into the screen; or you can use scroll bars. By far, the best option is to automatically sync the resolution to that of the primary computer.

All in all, MaxiVista allows you to connect two computers together and use the screen of the second one with the first. Still, in doing so, you are powering a whole computer just to use its display. It is highly inefficient in terms of power consumption. Thankfully, laptops are very power efficient.