Iconifer

Iconifer ... Helpful OS X Dashboard widget for converting images into iOS, Mac, and/or Windows icon formats. Automatically generates iTunes, iPhone 4, iPad, and iPhone PNG files, or outputs ICNS or ICO files with all necessary sizes.

Requires ImageMagick and libicns command line tools (can be installed via MacPorts).

iOS: 512x512 (iTunes store), 114x114 (iPhone 4 and iPod Touch w/ Retina displays), 72x72 (iPad), and 57x57 (older iPhone and iPod Touch models)

Mac: 512x512 ICNS (along with 256x256, 128x128, 32x32, and 16x16 sizes)