7Zip is a free open source file archiver/unarchiver that handles Zip files and many other formats. It is completely free to use on a personal computer or a commercial computer.

Features:

• No registration or nag screen
• Supported formats: ZIP, 7z, GZIP, BZIP2, TAR, ARJ, CAB, CHM, CPIO, DEB, DMG, HFS, ISO, LZH, LZMA, MSI, NSIS, RAR, RPM, UDF, WIM, XAR and Z
• Right-click integration with Windows Shell
• Command line version
• Pretty darn fast

Supports Windows, Linux, and also Mac OSX (sort of)

You can find 7zip here