I already have a two monitor system. I’d probably have more if my video card could handle it. However, when my fiancee told me about an app for the iPad that allows you to use the iPad as another monitor for your desktop, I had to check it out.
iDisplay consists of an app for the iPad and a desktop application that can be downloaded here.  It works with PC or Mac systems, and after some initial set up, your iPad will function as a second (or third, in my case) screen for your desktop computer.
After installing it, we played with it for quite awhile. My fiancee set the iPad up on my desk, dragged a Notepad into it, and typed at me for a few minutes – there’s absolutely no delay at all. One of the best features is that if other computers have the iDisplay application installed on them, on the same network as your main computer, you can link your iPad to those computers as well. One at a time, of course. But this can allow someone from another room to IM to you “Hey, I want to show you something. Connect to my computer.” And you do, and they simply drag whatever it is they want to show you onto what their computer is reading as a third monitor – even though you’re in a completely different room in the house from them.
There doesn’t appear to be a limit to the number of computers it can remember and connect to once the initial setup is finished. The installation of the desktop software DOES require that you restart your computer after installing it, however. So if you want to play with this right away, you’ll need to be prepared for that.
iDisplay seems to have limitless potential. I actually used it as a “remote” of sorts for my VLC media player last night. I had the player full screen, I detached the controls and dragged them onto my iPad screen, then sat back away from the computer and watched my movie, with the controls literally at the touch of my fingertips rather than at the fumble of a mouse. I’m sure there are many more uses for it that I haven’t even begun to tap into. And at $1.99 for the app, with the desktop application being free, I highly recommend trying it out for yourself.
Price: $1.99 (Requires free Desktop application  to work as well.)
Version reviewed: 2.4.7
Requires iOS 3.0 or later
Download size: 2.3 MB