Icon Tool for Developers
Mac OS Developer Tools
Easy icons for iOS, OSX and Android
You can build from a single large file - or provide multiple files and let Icons Tool fill in the gaps.
iOS - save as XCAsset or plist
OSX - save as XCAsset or .icns file
Android - Just save!
Icns Builder relies on iconutil which should be installed at /usr/bin/iconutil to generate an icns file.
If it isn't present, then installing XCode and clicking on 'Install command line tools' should install it for you.
It's cheap, It's simple - What's not to like?
Updated with latest icon sizes (iPad pro)