Icon files with the “2x” part are meant to be shown on a Retina display. Apple expects you to create icons with less details for use on Retina screens instead of just using the standard bigger ones. Icon Composer 2x is a replacement for Apple's Icon Composer that supports Retina Resolution Icons.
Small application to create CS5 and Web 2.0 style icons.
This tool helps you create icons for Mac OS X and iOS applications.
Icon Cast offers over 1,000 beautiful icons for web designers...
Native icon editor for Mac OS X icons,
Icon set completely free-to-use for commercial or personal applications.