For years, I have heard the question “can I use my old iMac as a screen for my laptop?” or variants of it, and the answer has always been no, an iMac is a full computer, and you can’t use it as a dumb display.
That was until I discovered ScreenRecycler, a brilliant application which makes possible the formerly impossible. You just run it on your primary Mac, and then on the Mac you want to use as a secondary display, you run JollysFastVNC, a free, excellent remote control application by the same author. (I also can’t sing the praises of that title enough, either; I use it all the time.)
ScreenRecycler and JollysFastVNC find each other automatically, and then before you know it, you’re using one Mac just as if it were an attached display on the other. I love it when smart people create cool things like this.