Icon Glue is the easiest way to quickly create an Apple Icon Image (.icns) for your Mac app from a single 1024x1024 pixels sized PNG or TIFF image. Just open your image, select what you want to create (Icon Glue will remember these settings) and save it. You can also open an OS X application bundle to easily extract its icon withouth having to browse through the bundle's internal folder structure.
Icon Glue can:
Rectangular (non-square) or too small images will be adjusted accordingly to ensure that the result is always a valid, Retina-compatible .icns image file.
Note: The downloadable demo is version 1.4.3; the version available for purchase on the Mac App Store is version 1.4.5.