AppleCommander is a cross-platform set of tools used to manipulate Apple ][ disk images. There is a GUI packaged for Windows, Linux, or Mac OS X, and there are a number of command-line tools that are useful for build chains.
Please see AppleCommander site for usage details.
Developer information has some details useful if you wish to contribute to AppleCommander!
Contribution information is a collection of people who have contributed to AppleCommander.