Unison has support for that on some platforms. On Linux, the option "-repeat watch" uses inotify to watch for changes and rerun unison each time there's a change. You can combine that with -auto and some of the options specifying policies for resolving conflicts (e.g. -prefer newer) to get a setup that works without user interaction.

Like Syncthing?

Also Resilio Sync, formerly known as BitTorrent Sync.