Some people can't live without at least two monitors on their work systems, and others find them only useful for particular tasks. Air Display 2 for the iPad can turn the tablet into an additional display for Macs and Windows PCs, and lets you add three more for maximum screen real estate.
Air Display has been around for years, and the second generation app Air Display 2 takes it even further. In its simplest use case, you install Air Display 2 ($9.99) on the iPad, download and install the host app on the Mac or PC, then select the iPad on the host system. This connects the iPad to the host via wi-fi (both must be on the same network), and sets it as an external monitor for the host.
The iPad can be arranged on the host just like any external monitor. In the case of an iPad with Retina Display, true resolution can be toggled on instead of the normal resolution handling that most users will find useful. The iPad can be set to connect to the host automatically when detected. The iPad can be used to either mirror or extend the host's desktop. There is also a presentation mode to disable the onscreen keyboard and touch control of the iPad.
Air Display 2 can be used with four iPads as additional monitors, although I've only used it with one or two. The setup works fine using the MacBook Air with an iPad Air and iPad mini (non Retina display). Performance is good with only occasional lags when the network is slow.
The iPads work the same as any external monitor, and app windows can be dragged to them at will. A nice feature is that the iPad's touch screen works as expected, effectively adding touch control to OS X.
Using the iPad as a second monitor is not practical all the time, but it comes in handy on occasion. A common use for me is when using OS X apps on the MacBook Air that have palette menus. With Air Display 2 I can drag the menus to the iPad while keeping the main window free on the MacBook. I also sometimes use it to keep my email open on the iPad and always visible while working on the laptop.
Air Display 2 is supposed to work with both OS X and Windows 7/8, but I've not been able to get it working on my ThinkPad Tablet 2 with Windows 8.1. It doesn't handle the iPad's resolution very well and regularly crashes my system. I suspect Air Display 2 needs to be updated to handle Windows 8.1, as I've used the first generation version with Windows 7 in the past without issues.
The developer claims Air Display 2 works on the iPhone, but I haven't tried it, and frankly I'm not sure how practical that might be. The app is $9.99 in the app store.