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.
Dar is a shell command that makes backup of a directory tree and files. Its features include splitting archives over several files, DVD, CD, ZIP, or floppies, compression, full or differential backups, strong encryption, proper saving and restoration of hard links, extended attributes, file forks, Door inodes, and sparse files, remote backup using pipes and external commands (such as ssh), and rearrangement of the "slices" of an existing archive. It can run commands between slices, before and after saving some defined files or directories (for a proper database backup, for example), and quickly retrieve individual files from differential and full backups. Several external GUIs exist as alternatives to its CLI interface, like kdar, DarGUI, SaraB, etc.
bzip2 is a portable, lossless data compressor based on the Burrows-Wheeler transform. It achieves good compression and runs on practically every (32/64-bit) platform in the known universe. The compression engine is also available as a library (libbz2), should you desire a programmatic interface.
storeBackup is a backup utility that stores files on other disks. It's able to compress data, and recognize copying and moving of files and directories (deduplication), and unifies the advantages of traditional full and incremental backups. It can handle big image files with block-wise changes efficiently. Depending on its contents, every file is stored only once on disk. Tools for analyzing backup data and restoring are provided. Once archived, files are accessible by mounting file systems (locally, or via Samba or NFS). It is easy to install and configure. Additional features are backup consistency checking, offline backups, and replication of backups.
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.
UnZip will list, test, or extract files from a ZIP archive, commonly found on DOS and Windows systems. A companion program, Zip, creates ZIP archives; both are part of the Info-ZIP project. Both programs are compatible with archives created by PKWARE's PKZIP or Nico Mak's WinZip, but in many cases the program options or default behaviors differ.
Star is a very fast, POSIX-compliant tar archiver, maintained for more than 30 years. It saves many files together into a single tape or disk archive, and can restore individual files from the archive. It includes command line interfaces for the "tar", "Sun-Tar", "cpio", "pax", and "gnutar" command-line syntax. It includes a FIFO for speed, a pattern matcher, multi-volume support, the ability to archive sparse files and ACLs, the ability to archive extended file flags, automatic archive format detection, automatic byte order recognition, automatic archive compression/decompression, remote archives, and special features that allow star to be used for full and incremental backups. It includes the only known platform independent "rmt" server program.
S3QL is a file system that stores all its data online. It supports Amazon S3, Google Storage, and OpenStack and effectively provides you with a hard disk of dynamic, infinite capacity that can be accessed from any computer with Internet access. S3QL provides a standard, full featured Unix file system that is conceptually indistinguishable from any local file system. Additional features include compression, encryption, data de-duplication, immutable trees, and snapshotting, which make it especially suitable for online backup and archiving. The design favors simplicity and elegance over performance and feature-creep. Care has been taken to make the source code as readable and serviceable as possible. Solid error detection, error handling, and extensive automated test cases are provided.
Lzip is a lossless data compressor based on the LZMA algorithm, with very safe integrity checking and a user interface similar to the one of gzip or bzip2. Lzip decompresses almost as quickly as gzip and compresses better than bzip2, which makes it well suited for software distribution and data archiving. If you ever need to recover data from a damaged lzip file, try the lziprecover program.
GNU `tar' saves many files together into a single tape or disk archive, and can restore individual files from the archive. It includes multivolume support, the ability to archive sparse files, automatic archive compression/decompression, remote archives and special features that allow `tar' to be used for incremental and full backups. It also includes `rmt', the remote tape server (the `mt' tape drive control program is in GNU `cpio').