fat_imgen
fat_imgen is an utility that is used to create and modify FAT12 floppy images with. It is licensed under the Fair License.
Binaries
Windows and Linux users can download the latest stable binaries from the project's download page.
Source code
The source code can be checked out from http://subversion.assembla.com/svn/slipstream/fat_imgen/. It shows many aspects of working with FAT12 images.