Ddiff is a tool for creating lists of binary differences between debian packages. It's shipped with a companion tool, dpatch, which can apply the generated debdiffs to the original package to create the new version.
For a more detailed discussion of the general idea, you may wish to take a look at the original project announcement, the version 0.1 announcement, or the version 0.2 announcement.
Version 0.1 was the intial release, which included basic support for debdiff generation and application.
Version 0.2 included diff compression, source checksum support, and the ability to store gzip headers. (By storing the gzip headers, the recreated files now have the same checksum as the corresponding original.)
The current version is 0.2 which was released 1 March 2000.
All released versions are still available on the web.
Ddiff is maintained by Tom Rothamel <software page, or to Tom's top page.