Using an early 2009 15" macbook pro as a display for new, 13" retina macbook pro I just purchases a 13" retina macbook pro, and am loving it. However, I now have my old 15" macbook pro lying around, and was wondering if it is possible to use it as a second display for my new machine. I haven't found anything online, and considering how old my 15" MB Pro is, I have a hunch it might not be possible...
You can use the program ScreenRecycler to use another Mac (or even a PC) as a second monitor. 

http://www.screenrecycler.com/ScreenRecycler.html