mergerfs is a union filesystem for FUSE. It allows to join multiple source directories, declare priority mappings for file operations, to provide a joined view to the underlying files. It's similar to mhddfs, unionfs, and aufs; but allows for accessing standard and extended attributes.
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 back
Gfarm is a network shared filesystem and alternative to NFS, but optimized for reliability, scaling and access from wide area network. It can federate local disks of network-connected PCs and compute nodes in several clusters, and supports explicit replica location control by users. Plugins exist for FUSE, Zabbix, Hadoop, Samba, MPI-IO, GridFTP, and a Linux kernel module is under development.
FUSE for OS X provides foreign filesystem support under Mac OS X. It's the successor to MacFUSE. And like the Linux variant it provides a user-land filesystem driver API, which is easier to develop for. Command-line tools are provided, and a kernel module which facilitates binding drivers like sshfs, procfs, AccessibilityFS, GrabFS, LoopbackFS, SpotlightFS, and YouTubeFS to the file system.