BTSync is a new application by the folks at BitTorrent that keeps folders between different computers synchronized. This is something you may be using Dropbox for at this time.
* No central server required
* No size limitations
* Supports more architectures (My Raspberry Pi can run it)
* Better network utilization (computers on a LAN find each other)
* Everything that makes the bittorrent protocol awesome.
* Secure. Copies of your files do not exist on cloud servers that goons have access to.
* Very easy-to-use sharing model.
* Syncs any folder with any set of machines you like
* no web interface to access your files (unless you build it)
* no phone apps (yet)
* if your ISP throttles torrent traffic (hello, comcast!), it will likely throttle this as well.