zpaq is a free and open source incremental, journaling command-line archiver for Windows, Linux and Mac OS/X.
Documentation can be found at: http://mattmahoney.net/dc/zpaqdoc.html
The ZPAQ archive format is described by a specification - http://mattmahoney.net/dc/zpaq206.pdf . The compression algorithm is described here - http://mattmahoney.net/dc/zpaq_compression.pdf .
The source code includes the libzpaq API providing compression and decompression services for applications in C++. Developers may be interested in the zpaqd development tool and sample configuration files found on the utilities page - http://mattmahoney.net/dc/zpaqutil.html .
zpaq is written by Matt Mahoney and released to the public domain. It includes code from libdivsufsort 2.0 (C) Yuta Mori, 2003-2008, MIT license, public domain code for AES from libtomcrypt by Tom St Denis and public domain code for salsa20 by D. J. Bernstein.
When compression ratio is important. ZPAQ is based on the top ranked PAQ context mixing algorithm.
zpaq.exe is a ZPAQ compatible multithreaded compressor and decompresser with 4 compression levels. The latest version can be found at http://mattmahoney.net/dc/zpaq.html
For quick help, run zpaq with no arguments.
For detailed usage, see zpaq.cpp.