The syncthing-inotify package uses the system to notify on file and directory changes rather than scanning the whole directory every couple of minutes. Makes a good compliment to the syncthing package.
Homepage - https://github.com/syncthing/syncthing-inotify/releases/tag/v0.8.3
Is it Open Source - yes
Link to source tarball/zip file - https://github.com/syncthing/syncthing-inotify/archive/v0.8.3.tar.gz (GitHub auto generated tarball. All releases look to be compiled binaries)
(Adding JoshStrobl since he's been working on the other syncthing stuff)