ARC (FreeArc)
From Just Solve the File Format Problem
FreeArc is a full-featured archiving application, developed by Bulat Ziganshin. Its native .arc format is unfortunately not compatible with any of the other ".arc" formats.
Its official site (when it still existed) featured downloads of Windows and Linux versions of FreeArc, documentation, and a downloadable set of source code which might be of use to somebody attempting to figure out the details of the file format, since the documentation doesn't give this sort of detail.
