PAQ is a series of data compression archivers that have evolved through collaborative development to top rankings on several benchmarks measuring compression ratio (although at the expense of speed and memory usage). The best compression in this series on most benchmarks is obtained by PAQ8JD, developed jointly by Matt Mahoney, Alexander Ratushnyak, Serge Osnach, Przemyslaw Skibinski, and Bill Pettis, and released December 30, 2006. It is surpassed on some benchmarks by WinRK in PWCM mode by Malcolm Taylor, released in January 2005. PWCM (PAQ weighted context mixing) is an independently developed closed source implementation of the PAQ algorithm. Specialized versions of PAQ have won the Hutter Prize and the Calgary Challenge. PAQ is free software distributed under the GNU General Public License.