iConvert is a free Web-based application that converts images and icons to 32-, 8-, and 4-bit icons system icons for Mac OS X, Windows, and Linux.
Mac OSX .icns icons can be converted and used on Windows. Windows .ico icons and even .cur cursors can be converted to Mac-compatible icons. Both can be made into Linux icons. It will also convert your image files to icons, to use in interface designs for all platforms, or simply to use as folder icons on your desktop!