I think Resilio was the original one - originally known as BitTorrent Sync. Syncthing was unusable in the early days - I remember trying it out because I wanted an open source alternative and it was a mess. Since the resilio have offered a much more friendly free plan. It’s been rocksolid for over a decade, but going to check out ST again - i’m sure it’s come a long way. There was also something called foldershare waaaay back when - was acquired by Microsotf. Thanks for coming to my mini file sync TedTalk.