Unison efficiently and reliably syncs my website (static blog mostly) between OS X and FreeBSD. It's one of those extremely underrated pieces of software that just quietly does an excellent job, yet most people never heard of it and continue to struggle with rsync or similar (syncthing fulfills a somewhat different role). Perhaps because it's written in an oddball language, or because the website isn't flashy enough.

I often evaluate a wide range of software before choosing what I consider best for a job, and many years ago, Unison came out way ahead in such an evaluation. It never failed me.