Ok, I may be exaggerating about the 2X boost in productivity, but DisplayPad has been one of the most useful apps I have purchased recently. With DisplayPad ($4.99), you can set up your iPad to serve as a portable secondary display for a laptop. When working with programs that have a lot of settings panels, such as Photoshop and other Adobe software, having a second display can save you a lot of time.
The app is really simple to set up and use. You will need to download a helper program that will run on your Mac. To start using DisplayPad, click on the helper program’s menu bar icon and choose your iPad (both devices need to be on the same Wifi network). Your iPad will then act just like any secondary display. Tapping with one finger will be the same as clicking with the mouse, and tapping twice will be the same as right-clicking. You can also drag with two fingers to scroll, just like on your laptop’s trackpad. I have been very happy with the performance. There is very little lag when you drag windows from your laptop screen to the iPad. I have even used DisplayPad with my regular secondary display to create a triple-display option when I use my laptop at home.