http://mattmahoney.net/dc/zpaq.html--official link
ZPAQ is a proposed standard format for highly compressed data that allows new compression algorithms to be developed without breaking compatibility with older programs. It is not patented. Software is open source (GPL v3) except for the libzpaq API, which is public domain.
* libzpaq 2.02 - Contains the ZPAQ library API, ZPAQ specification, reference decoder, the zpaq archiver, zpipe file compressor, and zpsfx self extracting archive stub. To install: create a folder c:\zpaq and unzip all the files there. See enclosed readme.txt for further instructions. Last updated Nov. 13, 2010.
The ZPAQ specification and reference decoder define a format for highly compressed data. It is designed so that when better compression algorithms are developed, archives compressed with older verions can still be decompressed with the newer version and vice versa. This is achieved by storing the decompression algorithm in the archive header. Algorithms are based on the PAQ context mixing model with optional postprocessing.
libzpaq is an application programming interface in C++ for reading and writing ZPAQ compressed data to or from files or objects in memory. This part of the software is public domain.
Download latest version
http://mattmahoney.net/dc/libzpaq202.zip
