Matthias Kretschmann has developed a template for designing your own icons for the admin area of WordPress. This is very useful for plugin and theme developers looking to add new icons to their new menus.
Icons are also used when you have custom post types on your blog. The template contains a total of 6 icon sizes and is available in PNG and PSD formats.
The download package also has some PHP code that you can include in your plugin or theme to get the icons working in your WordPress Admin area.