I recently purchased a MacBook Air and iPad Mini. After doing some research of how I could utilize the iPad mini as a second screen to my MacBook Air, I came across iDisplay.
For those of you who may not have heard about iDisplay, it allows up to 35 devices, via Wi-Fi to be used as external screens (mirrored or independent). I purchased it a few days ago and have been very impressed with it.
I often tether my MacBook and my iPad to my iPhone and I can then use the iPad as a separate "monitor" for photoshop palettes, Lightroom's second display or any other app that I wish to display on there.
So far it seems like a fantastic product which I enjoy using and haven't found any real annoyances or instability issues with.
It also has a Windows client should you want to extend your screen from there. iDisplay can also be used on your iPhone to be used as a remote control for various applications running on a computer.