RioFS is a userspace filesystem for Amazon S3 buckets that runs on Linux and Mac OS X. It supports versioned and non-versioned buckets in all AWS regions. RioFS is intended as a storage backend for legacy daemons that cannot talk natively to S3. It handles buckets with many thousands of keys and highly concurrent access gracefully.
dantalian is a transparent file-tagging system using hard links. Tags (which are directories) can be accessed on the file system level (with or without fuse), allowing maximum interoperability with other applications. Tagging operations are performed with scripts, and can also be hooked into other applications. dantalian is file-agnostic and can be used with any and all types of files. Tags can be organized hierarchically and can also be tagged. With (optional) fuse support, dantalian can also create virtual tags (currently limited to tag intersections (AND), but will be expanded later), which can be accessed transparently on the filesystem level.
GitFS is a fuse- and git-based remote filesystem with local cache for disconnected operation. It does periodic commits and pushes/pulls to keep a local shadow file system in sync with a remote repository. The filesystem should continue to work when you are disconnected. Merging after disconnected updates from multiple locations is left to git.
QueryFS is a FUSE-based virtual filesystem that is based on queries. There are plugins to parse various query syntaxes and each query creates a virtual directory. Currently the most elaborate query language is SQL-based; you can either provide access to data stored in an SQL database, or keep an index of files in the SQL database and generate search directories with symlinks to found files. QueryFS and its SQL2 query DSL fully support creation of refinable searches: for example, the author has an index of downloaded email files; one can see all the distinct senders registered in the DB and then go into a directory of one sender and see all accounts that ever received an email from this sender.
SIMPLE-MTPFS (Simple Media Transfer Protocol FileSystem) is a file system for Linux (and other operating systems with a FUSE implementation, such as Mac OS X or FreeBSD) capable of operating on files on MTP devices attached via USB to local machine. It allows the end user to seamlessly interact with MTP device files.