Macintosh icons should be saved with an .icns file extension. You can create, edit, and save Macintosh icons with a developer tool like Icon Composer or third-party tools that work with your graphics programs, such as the Iconfactory's IconBuilder plug-in for Photoshop.
As mentioned above, Windows icons expect an .ico file extension. Iconfactory.com and Aha-soft are but a few of the extremely useful resources available for creating and editing Windows icons. Make sure you take a look at our compare-and-contrast evaluation in next weeks article: Creating Your Own Icons Part 2.