Free Compression Libraries

TurboPower Abbrevia
A compression toolkit for Delphi, C++Builder, Kylix, and Free Pascal, supporting compressing/decompressing PKZIP, Microsoft CAB, tar, gzip, and bzip2 archives, and can create self-extracting executables. On Windows it also provides Delphi wrappers for the LZMA, Bzip2, and WavPack SDKs, and PPMd decompression. Includes TreeView and listview controls.

Basic Compression Library
A set of open source implementations of several well known lossless compression algorithms, such as Huffman and RLE, written in portable ANSI C, completely independent of system functions, it can be used in almost any system, ranging from Windows, Mac OS X and Linux-systems to embedded systems.

libmspack currently contains a complete library infrastructure and complete CAB, CHM, SZDD and KWAJ decompressors.

An Objective-C class to compress or uncompress zip files, According to the author it can be used for iPhone application development, and cocoa on Mac OSX as well.

zlib is designed to be a free, general-purpose, legally unencumbered lossless data-compression library for use on virtually any computer hardware and operating system.

The UniquE RAR File Library is a platform-independent, small and fast static library for decompressing RAR files (RAR archives are created with WinRAR).