A few days ago my mini-display to DVI adapter that connects my external display to my iMac stopped working. Whilst waiting for a replacement I wondered if my iPad could act as a secondary display in the meantime. If you are like me then you will feel as if one of your arms has been cut off when you have to work on a single display - it feels totally inadequate despite a size of 27" :-)
A few minutes after posting a question about such a setup on Twitter I had received several recommendations for an app called Air Display (not to be confused with Adobe AIR, it has nothing to do with that at all). I bought the app and tried it - it turns out that it works very well indeed and is also great to test some basic touch screen interactions, especially if you develop Flash applications for mobile. I also recorded the following short video to quickly demo the setup. If you have any questions please post them below in the comments.