Icon Builder is a Mac app that takes your images and creates all the required icon files for use in your iOS apps.
It now supports Xcode 5’s image assets folders and if you are using Xcode 5, it will create the icon sizes required for iOS 7 app.
Icns Maker is a Mac app that takes your images and creates icons for use in your Mac apps.
For Xcode 4 or earlier, it creates a .icns file.
For Xcode 5, it creates a folder of images for use in your project’s image assets folder.