libferris is a virtual filesystem that exposes hierarchical data of all kinds through a common C++ interface. Access to data is performed using C++ IOStreams and metadata is available as key-value pairs through the Extended Attributes (EA) interface. Rich support for filesystem indexing is included to provide timely search results for well into millions of files. Ferris uses a plugin API to handle a large range of data sources, metadata, and index and search strategies. Filesystems include file:// with monitoring, XML (mount an XML file as a filesystem), relational databases, ISAM databases (Berkeley db, tdb, gdbm, eet et al), xmldb, LDAP, Applications (Evolution, Firefox, Emacs), HTTP, FTP, sockets, and RDF (from XML, binary, soprano). EA generators include image, audio, and animation decoders.
Ceph is a distributed network file system designed to provide excellent performance, reliability, and scalability (from terabytes to exabytes). Data is stored in the form of "objects" (variable length, named chunks of data) in a distributed and fault tolerant cluster of OSDs (object storage devices--servers running the Ceph OSD daemon). Metadata (the file system namespace) is managed by an independent cluster of metadata servers (MDSs), which dynamically repartitions the directory hierarchy in response to the current workload.
libguestfs is a set of tools for accessing and modifying virtual machine (VM) disk images. You can use this for viewing and editing files inside guests, scripting changes to VMs, monitoring disk used/free statistics, P2V, V2V, performing partial backups, cloning VMs, and much more. libguestfs can access nearly any type of filesystem including: all known types of Linux filesystem (ext2/3/4, XFS, btrfs, etc.), any Windows filesystem (VFAT and NTFS), any Mac OS X and BSD filesystems, LVM2 volumes, MBR and GPT disk partitions, raw disks, qcow2, CD and DVD ISO images, SD cards, and dozens more. libguestfs doesn't need root permissions.
EncFS is an encrypted pass-through filesystem which runs in userspace on Linux (using the FUSE kernel module). Similar in design to CFS and other pass-through filesystems, all data is encrypted and stored in the underlying filesystem. Unlike loopback filesystems, there is no predetermined or pre-allocated filesystem size.
Cromfs is a compressed read-only filesystem for Linux. Cromfs is best at archiving gigabytes of big files that have a lot of redundancy. It aims primarily at achieving a strong compression, even at the cost of memory and CPU time resources. It uses the LZMA compression algorithm from 7-zip and block merging.