lsyncd is somewhat like Dropbox but very simple. Essentially, it's a daemon that listens for directory changes via inotify and automatically triggers an rsync if needed to synchronize directory trees. Since you can sync any directories with it and can also intervene in the sync process via Lua integration in lsyncd, it could be useful for some loosely coupled sync situations (e.g., autonomous nodes in a very loosely coupled cluster or home servers that automatically sync to a server on the internet). Additionally, it offers functions similar to Hazel - you can assign various actions (not just sync) to different file change events.