It took me a while to finally get to this application, however I have to say that Duet Display by Kairos Technologies is an absolute joy to use.
Developed by a team of ex-Apple engineers, Duet is an application that allows you to turn your iPad into an extra display - and for those of you fortunate enough to have an iPad Pro, this offers some serious functionality.
There are two core components to the product. Firstly, there is an iOS application that needs to be purchased (usually £11.99, however it is currently 50% off and you can get it here. Secondly, you need the software that runs on your Mac/PC (downloadable from here.
Once you open the iOS application, simply connect up your iPad with either a Lightning or 30-pin cable, dependant on your iPad model and you have a very handy second screen.
I decided to purchase this when I was trying to marry up an OmniPlan 3 project with it's associated tasks in OmniFocus. Split-view is great to a point, but I have so much detail in the task actions that I really needed to use a second screen - yet being a largely mobile consultant, I can't really carry one about with me. Now, my iPad travels with me wherever I go as a second screen and I can safely say it's the best £6 I've spent in a long time. The quality of the display on an iPad Pro is amazing (you can set different resolution settings as well if your eye-sight isn't too great) and I was happy with some of the dropped-jaws my colleagues had when they saw it in action for the first time.
Well worth a look.