There are some studies that claim that productivity is increased manifold by a second or more monitors. Whether you buy that or not, it might be pretty handy at times to have that setup as well. But what if you don't have a spare monitor, can't afford one, or can't justify buying one? If you have an iOS device around, especially an iPad, then you're in luck. Duet is an upcoming app that will turn that those into an extended display for Macs.