Syncthing continues development, has a great gui now (use to be only browser based), is opne sourced and free, and has been awesome for me for a few months now. I was one of the many testers of Bit Torrent Sync (all the way back when it was in alpha) and I won't touch it again. They have made bad decisons, under-handed moved behind the scenes, and are not continuing the quality rhat they had 2 years ago. Ha!