Zzip
From Just Solve the File Format Problem
Zzip is a file compression/archiving utility (with Windows and Unix versions) and its file format. The program doesn't seem to have been updated since June 2001, at which time the "unix version doesn't work any more" according to the revision history. The revision history also gives version 0.33a as when the "multiple-files-in-a-single-archive feature added", so it was apparently a single-file archiver before that.
The source code is downloadable, so the file format could probably be figured out from that.
Zzip was designed to be used either as a standalone program or a library to embed in other programs.
Disambiguation
Not to be confused with zziplib, a library for ZIP format.
