paq8m is an open source (GPL) file compressor and archiver.
Last update Aug. 4, 2007 by Matt Mahoney.

Contents of paq8m.zip:

readme.txt - this file
paq8m.exe - Win32 (MinGW g++) executable for Pentium MMX and higher

paq8m.cpp - C++ source code (Aug. 4, 2007)
paq7asm.asm - NASM/YASM assembler code for Pentium MMX or higher
paq7asmsse.asm - NASM/YASM for Pentium 4 (SSE2) or higher in 32 bit mode
paq7asm-x86_64.asm - YASM for x86-64 bit processors (tested in 64 bit Linux)

paq8m can be compiled for other processors without the assembler
code using the -DNOASM option (but it will run slower).
The assembler code is the same for all paq7/8 versions.

paq8m was written by Matt Mahoney (as paq8f) with improvements by
Bill Pettis (based on improvements by Alexander Ratushnyak and
Przemyslaw Skibinski in the paq8hp* series) and Serge Osnach (additional
models), Andrew Paterson (Borland port).  The assembler code was ported 
to 64 bit by Matthew Fite and 32 bit SSE2 by wowtiger.

Other contributors to the PAQ project: Berto Destasio (tuning earlier
models for better compression), Johan de Bock (benchmarking, compiling
fast exectuables), David A. Scott (arithmetic coder improvements),
Fabio Buffoni (speed optimizations), Jason Schmidt (compression
improvements), Rudi Cilibrasi (text modeling), and Pavel L. Holoborodko
(PGM image modeling), Jari Aalto (licensing/distribution), and Jan Ondrus
(improvement to JPEG modeling).

This work would not be possible without the benchmarking efforts of
Marcus Hutter (Hutter prize), Werner Bergmans (maximumcompression.com)
Johan de Bock (UCLC), Berto Destasio (Emilcont benchmark), Stephan Busch
(Squeeze Chart), Leonid A. Broukhis (Calgary Corpus Challenge),
and Black Fox.

A similar (but rewritten) context mixing algorithm is used in
WinRK 3.0.3 (pwcm mode) by Malcolm Taylor.  Modified versions of 
PAQ (faster but less compression) are used in UDA and WinUDA by dwing,
and in xml-wrt by Przemyslaw Skibinski.

