Icontool for Mac & Win
Screenshot, Source: www.iconbuilder.com
"IconBuilder is a filter for use with Adobe Photoshop™ that makes creating icons a snap. By harnessing the professional power of Photoshop and Photoshop layers, IconBuilder provides icon artists with the most complete tool set possible for modern icon design."
I tested it and i think that it is the best icon-tool i have seen. It is much better than "gif movie gear" wich is recommended in the windows xp visual styleguide for creating icon-files.