fsprotect is a set of scripts that combine tmpfs and aufs to make existing filesystems immutable. After the filesystems are protected, everything that is written will be lost when the computer powers off. It is a great tool for testing and for public computers like those in schools, libraries, etc. It is also very easy to use. It is currently available only for Debian-based systems.
Hgfs is a read-only filesystem interface to Mercurial repositories. The interface gives access to the commit message, manifest, and files of each revision, and to .tgz's of each revision (the .tgz's are generated as they are read). The filesystem is a front-end for the Mercurial library that comes with it. All code is written in Limbo, for Inferno.
FSter is a virtual filesystem implementation based on FUSE and exploiting Tracker's indexer to allow access to files according to the metadata with which they’re associated. Its behavior is highly customizable with XML configuration to describe in detail the desired hierarchy of files.
Deduplicator is a simple and efficient data deduplicator that works by hard linking files that have the same content. It is ideal for reducing the size of backups. It can save and restore intermediate results, so you can run it in a few short intervals, and allows you to review changes before they are committed to disk.
Chnorm is a simple command line utility for setting the owner, group, and the mode of given files and directories on a per file/per directory basis, recursively. It is especially useful if you often copy files with brain-dead permissions from filesystems not supporting the Unix permission scheme. It detects executables based on their contents and sets their permissions accordingly.