Syncthing: Open Source Dropbox and BitTorrent Sync Replacement
Since we're listing alternatives here: I setup SeaFile (http://seafile.com/) a couple of months ago and I'm loving it so far. Mainly because it has client-side encryption and allows a single client to sync with different servers and selectively choose which 'libraries' (basically directories that are under sync) to use. Typical usecase is having a personal server for personal files and another one at the office for work-related stuff.