[wlesieutre]

Syncthing is the other option I've used. Setup isn't as easy as Bittorrent Sync though (each computer has to have the node IDs for other computers added manually). And last I checked, there's not a convenient "run in the background" checkbox, so it's not well suited to non-techies yet. But it's got open source going for it. Don't know if it's been subjected to any security audits, but the fact that it eventually could be puts it ahead of Bittorrent.

http://syncthing.net/