One very popular and fun iPhone / iPad app that I love to use is Avatron Software's Air Display. As we described in a First Look back in May of 2010, Air Display turns an iOS device into a small wireless monitor that can be used to display app windows from a Mac or Windows computer.
Now Avatron has made Air Display even more useful by creating a Mac app to do the same thing. Air Display for Mac (US$19.99 in the Mac App Store) is a bit more expensive than its iOS sibling, but it's also more capable than the mobile device version -- especially if you have a second Mac with a large screen.
Similar to Air Display for iOS, Air Display turns a Mac into a wireless display for extending the desktop of your other Mac or Windows computer. Air Display is available in the Mac App Store, and needs to be installed on the Mac that you want to turn into a secondary display. You then need to visit the Avatron website to download the host software for your primary computer. As noted, that computer can be either a Mac (running Snow Leopard) or a Windows PC (running XP, Vista or 7). You do have to reboot the host computer during the installation process. The same host software works with both iOS and Mac extended displays.