The BitTorrent Blog describes how to get BitTorrent Sync running on FreeNAS.
So you’ve upgraded to a new computer. Congratulations. Now you have to decide what to do with that old computer. Give it to your parents? Reenact your favorite scene from Office Space? How about you turn that piece of junk into a Network Attached Storage (NAS) device running BitTorrent Sync. It may not play Crysis, but it will provide you with a plethora of options including FTP, redundant storage, and most importantly, BitTorrent Sync.