Creating App Icons in all the required sizes for a Mac app can get pretty tedious. Here is a Photoshop action to simplify the task:
The first thing it does is to paste the clipboard contents into a square document — so it's designed for workflows where you edit your artwork in Illustrator, and simply copy the shapes to the clipboard when you're ready to generate some PNGs. The Photoshop action will generate transparent PNGs at 1024, 512, 256, 128, 64, 32, and 16 pixels square.
I use bilinear downsampling from the original 1024 pixel base image to generate each icon. This results in nice crisp edges without ringing (sharpening halos).