FUSE isn’t really a stackable filesystem, but it can be used as one.
- Dokan, FUSE for Windows: http://dokan-dev.net/en/
- FUSE for OS X (MacFuse successor): http://osxfuse.github.com/
- Fuse4X (MacFuse fork): http://fuse4x.github.com/
Also, maybe fanotify could be used for this, which is more limited but faster (?) than inotify. The underlying system of all of these is fsnotify.
Of course, these are all for Linux.
Watchdog, in Python, is a cross-platform system used to monitor filesystem events. https://github.com/gorakhargosh/watchdog. It uses inotify on Linux, FSEvents and kqueues on Mac, ReadDirectoryChanges on Windows).
Article on IBM DeveloperWorks about Linux VFS: http://www.ibm.com/developerworks/linux/library/l-virtual-filesystem-switch/