If you've ever wondered what software I use to create the ICO and ICNS files to create my icons, here's the answer: Axialis' Icon Workshop!
After drawing and painting the icons so that they look as official-like as possible on Photoshop, I save them as a PNG file with a transparent background (which you also get when you download the icons) and then run them through Icon Workshop, which is an amazingly simple and straightforward piece of software.
Just import the PNG file, push the "create a Windows icon file" or the "create a Macintosh icon file", save, and you're done! There's a lot of powerful dials and tools for the experienced users, too!
So if you're looking into designing some icons for the OS of your choice, drop by Axialis' page and try Icon Workshop out. I'm sure it's exactly what you're looking for!
Icon Workshop