If you’re trying to promote an app that you’ve created, or trying to design an icon for an app and you want to see what it looks like, then this tool is what you need. It’s free to use, and a great resource to create some fun images to add to your site.
To help you actually create the icon in the first place, there’s a handy tutorial on MacInstruct.
To take your icon file and “polish” it so that it looks like it will in the app store, you can use this fantastic tool to upload your rough icon, and download a polished one!
For example, when I was creating an icon for SPARK! and SPARK HD for Tamara Vukusic at Spark Your Imagination Inc. I wanted to see what they would look like finished. What we started with, then flat finish, and finally glossy finish.