OverlayFS is a FUSE filesystem module that merges content of several directories in to a single directory transparently.. Linux VFS Tour Unguided - with LXR and TiddlyWiki.
OpenGFS is a clustered filesystem. It's usefulness is thought to be mostly in SAN's, but goes well beyond that. It can be used with iSCSI, HyperSCSI, and even firewire.
OpenGFS is undergoing many changes at the moment, we appreciate any help ... OCamlFuse
OcamlFuse project is an ocaml binding for FUSE virtual filesystem. It enables you to write your own multithreaded, efficient userspace filesystems using the ocaml programming language.. SourceForge.net: ocamlfuse.