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').
UDPCast allows to send data simultaneously to many destinations on a local net. This can for instance be used to install entire classrooms of PCs at once. The advantage over using other methods (nfs, ftp, whatever) is that udpcast uses Ethernet's multicast abilities: it won't take longer to install 15 machines than it would to install 2. The tool comes with a busybox bootdisk for easy loading of the tool. However, udpcast can also be started from the command line of an already installed system, and can be used for other purposes than just system installation.
Unarc unpacks an archive and creates a top-level directory to unpack into if it's needed. Unarc works with lots of archives, not just tarballs (bz2, zip, and even rpm are supported). There is a companion program arcdir which provides a uniform way to archive directories into tarfiles, zipfiles, etc.
The URARFileLib is a small static library that allows you to read files from RAR archives created with RAR and WinRAR. Listing, decompression, and decryption with full RAR 2.0 compatibility is done directly in your application, so there is no need for a DLL or any other external file. It is based on the free unRAR source code by Eugene Roshal, and designed for easy but powerful usage in demos and intros. It is also useful if you want to port your programs since the it is written in pure ANSI C (some parts are optimized in assembly) and supports multiple operating systems.
Unison is a file-synchronization tool for Unix and Windows. It allows two copies of a collection of files and directories to be stored on different hosts (or different disks on the same host), modified separately, and then brought up to date by propagating the changes in each replica to the other. Unison can deal with updates to both replicas of a distributed directory structure. Updates that do not conflict are propagated automatically. Conflicting updates are detected and displayed. Unison can communicate through a direct socket link or through an rsh/ssh tunnel. It uses network bandwidth efficiently.