Former Apple engineers Matt Ronge and Giovanni Donelli have created an absolutely amazing software solution called Astropad which magically turns your iPad into a professional graphics tablet for your Mac.
The iOS application talks to the Mac client via Wi-Fi or USB which then translates your stylus strokes on the tablet into corresponding actions in virtually any Mac app such as Adobe Photoshop, Adobe Lightroom, Pixelmator and many more.
Drawing on your Mac with your iPad works with most pressure sensitive styluses on the market. Your stylus strokes are transferred to the Mac in a silky-smooth sixty frames per second, thanks to a proprietary technology called Liquid that provides nearly three times more fluid responsiveness than Apple's AirPlay.