Copied from 7-Zip Homepage:


License

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. 

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. But you can make a donation to support further development of 7-Zip.

The main features of 7-Zip

High compression ratio in new 7z format with LZMA compression
Supported formats: 
Packing / unpacking: 7z, ZIP, GZIP, BZIP2 and TAR
Unpacking only: ARJ, CAB, CHM, CPIO, DEB, DMG, HFS, ISO, LZH, LZMA, MSI, NSIS, RAR, RPM, UDF, 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 74 languages
 

7-Zip works in Windows 98/ME/NT/2000/XP/Vista. There is a port of the command line version to Linux/Unix.

On 7-Zip's Source Forge Page you can find a forum, bug reports, and feature request systems.

Compression ratio

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 of other zip compatible programs.