puffs —
Pass-to-Userspace Framework File System
file-system PUFFS
pseudo-device putter
puffs provides a framework for creating file systems as
  userspace servers. The in-kernel VFS attachment is controlled through a
  special device node, /dev/puffs. People looking to
  implement file systems should use the system through the convenience library
  described in puffs(3).
A puffs file system can be unmounted regularly using
  umount(8). The file system will
  automatically be unmounted in case the userspace server is killed or the
  control file descriptor closed.
An unsupported experimental version of puffs first
  appeared in NetBSD 4.0. A stable version appeared in
  NetBSD 5.0.