Avatron's Air Display has been around for a while now on the App Store for the iPhone and iPad. The company has announced that a new version of the app is now on the Mac App Store for download as well. The Mac version allows any Mac computer to be used to extend the display of another Mac or Windows PC.
The app works pretty much the same as the iPhone and iPad versions using a wireless connection to make your MacBook a second display for your Windows computer or for your Mac desktop. This might be a cool way to get your Mac and Windows PCs to coexist on one computer.
Air Display for Mac can operate in full-screen and window modes and it can be connected over the Ethernet if you want making for a faster connection than WiFi. The iOS version of the app aren't able to do that. Avatron says that when you are connected to WiFi the mouse pointer movement is just as it would be on the home computer. The Mac app requires OS X 10.6 or higher and an Intel CPU to run. You can buy the app right now for $19.99.