AstroPad turns your iPad into a Mac graphics tablet, mirroring a section of your Mac’s screen when connected via WiFi or USB. Once connected you can sketch/draw on the tablet with an iPad stylus or finger. It will operate pretty much like a Wacom graphics tablet when you’re using a Desktop graphics app with a drawing tool like Pixelmator, Photoshop, or Sketchbook Pro.

http://astropad.com/