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.
Comments and Reviews
Excellent for incremental backups of large text files or folders. High compression ratio and fast for small changes. Can be used for versioning/snapshots (development). Another developer has forked it with some additional featured (franz zpaq). No real alternative found.
No good GUI are available sadly. Bandizip is the best UI for extracting files, but it's ad supported or $30 per user. That major con aside Zpaq is simply amazing. It's fast, compact, and resilient.
The #1 reason to sue ZPAQ is that you want to keep lots of iterations/revisions of the same file in case you want to restore to an earlier version.
I have over 500GB of backups compressed into 5GB space and it only takes a couple moments to add new files or extract something when I need it.
The only other compressor that works this well for my files is ARC, but it is crazy slow, and time consuming to extract compared to ZPAQ.