7Zip

7-Zip is an open source file archiver, or an application used to compress files. 7-Zip operates with the 7z archive format, but can read and write several other archive formats. The program can be used from a command line interface, graphical user interface, or with a window-based shell integration. 7-Zip began in 1999 and is developed by Igor Pavlov. The cross-platform version of the command line utility, p7zip, is also available. 7-Zip is open source software. Most of the source code is under the GNU LGPL license. The unRAR code is under a mixed license: GNU LGPL + unRAR restrictions.

7z

By default, 7-Zip creates 7z format archives with a .7z file extension. Each archive can contain multiple directories and files. As a container format, security or size reduction are achieved using a stacked combination of filters. These can consist of pre-processors, compression algorithms, and encryption filters. The core .7z compression uses a variety of algorithms, the most common of which are bzip2, LZMA2, and LZMA. Developed by Pavlov, LZMA is a relatively new system, making its debut as part of the 7z format. LZMA consists of a large LZ-based sliding dictionary up to 4 GB in size, backed by a range coder.[7] The native 7z file format is open and modular. All filenames are stored as Unicode. TopTenReviews found that the 7z compression is at least 17% better than ZIP,[9] and 7-Zip's own site reports that while compression ratio results are very dependent upon the data used for the tests, "usually, 7-Zip compresses to 7z format 30-70% better than to zip format, and 7-Zip compresses to zip format 2-10% better than most other zip compatible programs." The official 7z file format specification is distributed with the program's source code, in the 'doc' subdirectory.