bfsync is a file-synchronization tool that allows you to keep a collection of big files synchronized on many machines. It is built around a FuSE filesystem, so repositories can be mounted and the contents can be viewed and modified via the mount point. A collection of commands like commit/push/pull/put/get can be used to control bfsyncs behaviour, and trigger data / history transfers between machines. In addition to synchronizing files between many machines, bfsync can also be used to store backups.
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.
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.