From TereNovo Software:
NovoIcons is the best icon extractor. It's simple and intuitive, but has more features than the competition. It can extract icons and export them into PNG, TIFF, GIF, JPG, or ICNS. 512x512 icons are fully supported.
While this version is almost 3 years old, it still functions well. Version 2.0 is under way, but it is barely into development so it will be a long time until release.