SQX

SQX is an open-source archiver. The source code download includes a text file giving file format details. The source code is archived as a .sqx file itself, but fortunately an SQX extractor is also included as a standalone Windows .exe. (Bad luck for users of any non-Windows platform, though.)

Links

 * Wikipedia article
 * Official site
 * Total SQX plugin for Total Commander file manager
 * Some discussion of SQX