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7-Zip is a file archiver with a high compression ratio.

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. Check license information here: 7-Zip license.

You can use 7-Zip on any computer, including a computer in a commercial organization. You don't need to register or pay for 7-Zip.

Features:

High compression ratio in 7z format with LZMA and LZMA2 compression
Supported formats:
Packing / unpacking: 7z, XZ, BZIP2, GZIP, TAR, ZIP and WIM
Unpacking only: AR, ARJ, CAB, CHM, CPIO, CramFS, DMG, EXT, FAT, GPT, HFS, IHEX, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, QCOW2, RAR, RPM, SquashFS, UDF, UEFI, VDI, VHD, VMDK, WIM, XAR and Z.
For ZIP and GZIP formats, 7-Zip provides a compression ratio that is 2-10 % better than the ratio provided by PKZip and WinZip
Strong AES-256 encryption in 7z and ZIP formats
Self-extracting capability for 7z format
Integration with Windows Shell
Powerful File Manager
Powerful command line version
Plugin for FAR Manager
Localizations for 87 languages

The program's ability to shrink files was stunning. In our tests, 7z archives were up to 40 percent smaller then their ZIP equivalents, although compression did take longer, and the highest compression settings can hog system resources. Advanced options include the ability to create solid and self-extracting archives, and to adjust the compression level and password protection. The program also has a command line module. Though the main interface is overly simple and looks like it was designed for Windows 95, it's not hard to use and fairly self-explanatory. The context menu options, which include "testing" an archive, indicate that 7-Zip should be taken seriously.