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 zlib data format is itself portable across platforms. Unlike the LZW compression method used in Unix compress(1), the compression method currently used in zlib essentially never expands the data. (LZW can double or triple the file size in extreme cases.) zlib's memory footprint is also independent of the input data and can be reduced, if necessary, at some cost in compression.
pngcrush is an excellent batch-mode compression utility for PNG images. Depending on the application that created the original PNGs, it can improve the file size anywhere from a few percent to 40% or more (completely losslessly). The utility also allows specified PNG chunks (e.g. text comments) to be inserted or deleted, and it can fix incorrect gamma info written by Photoshop 5.0 as well as the erroneous iCCP chunk written by Photoshop 5.5.
CFL (Compressed File Library) is meant for the save-files used by games, and other sorts of data files. Its features include pluggable class factories, easy CFL file creation at runtime, and the ability to have multiple separate CFL objects in memory at the same time. The package also includes a CFL testing tool to make it easier to port the library and to create additional plug-in compressors, preprocessors, and ciphers. It also includes a makecfl utility, which makes it easy to create ini files.