If the monitor or built-in screen of your Mac it falls short and you need more work space but you do not want to invest in buying a monitor to expand that space, you can also use your iPhone or iPad to get that little extra that, as little as it may seem, may meet your expectations.
This would be achieved thanks to Duet Display, an application that has just appeared on the Mac App Store at a price of 8,99 Euros due to its launch and as I have said, I would be able to expand your desktop to iPad screen just connecting it by lightning cable or 30-pin via USB.
To configure the application, you don't really have to do anything special, just download the program for Mac for free and in turn do the same on your iPad or iPhone. It does not matter the configuration of your Wi-Fi, Bluetooth or Airplay network, since as I said before, the connection is made physically with the data cable.
According to the developer formed by ex-engineers who worked for Apple, the performance of the application would be excellent, that is, without lag or delays in the response of the touch screen, always at a refresh rate of 60 frames per second with the advantage to be Plug and Play in addition to support Retina resolutions optionally and securely wired connection.
In short, it seems that according to user reviews, it works perfectly in both OS X Mavericks 10.9 and OS X Yosemite 10.10, having as a requirement to have an iOS device with version 5.1.1 or higher, so if you are within that niche of users that require that little extra screen space, Duet Display is the best option for the quality / price ratio.