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.
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.