GNU xorriso creates, loads, manipulates, and writes ISO 9660 filesystem images with Rock Ridge extensions. Files can be copied in and out. The session results get written to optical media or to filesystem objects. Rather than needing external tools for ISO 9660 production and CD/DVD/BD burning, it is a static compilation of libburnia-project.org. Thus GNU xorriso depends only on fundamental operating system facilities.
Chiron FS is a FUSE based filesystem that implements replication at the filesystem level like RAID 1 does at the device level. The replicated filesystem may be of any kind you want; the only requisite is that you mount it. There is no need for special configuration files; the setup is as simple as one mount command (or one line in fstab).
Sshbak is very simple but powerful and secure remote backup utility. It performs a fast and comprehensible daily (usually) backup of multiple machines via remote SSH connections. The result is a simple set of tar+gzip files. It supports the backup of piped program output (allowing remote backup of mysqldump or pg_dumpall results), parallel backup of different machines, and more.
libburnia is a project comprised of libraries and binaries for reading, mastering, and writing optical discs. It provides libburn, a CD/DVD/BD burn library, libisofs, a manipulation library for ISO 9660 filesystems, and libisoburn, a ISO 9660 multi-session library. On top of them there is cdrskin, a cdrecord emulator, and xorriso, a all-in-one application for ISO 9660 multi-session operations which additionaly provides a limited emulation of mkisofs.
LRZIP is a compression program and library that can achieve very high compression ratios and speed when used with large files using unlimited sized compression windows. It uses the combined compression algorithms of zpaq and lzma for maximum compression, lzo for maximum speed, and the long range redundancy reduction of rzip. It is designed to scale with increases with RAM size, improving compression further. A choice of either size or speed optimizations allows for either better compression than even lzma can provide, or better speed than gzip, but with bzip2 sized compression levels. It also has high grade password protected encryption and full STDIN/STDOUT support.
fakedbfs is a suite of tools and a library for cataloguing files. Anything that is a file can be catalogued along with metadata. The metadata is then searchable through various querying facilities. It runs entirely in userland and was inspired by BFS, which doesn't do content indexing, only metadata.