BitTorrent Sync is what seems to be a great encrypted serverless Dropbox alternative. You can read more about the technical background here. It provides clients on OSX, Linux, Windows, Android, iOS, ARM and others. The client for ARM can easily be run on the Raspberry Pi. Just download the binary and copy it to your pi. Start it by executing “./btsync”. You can now access a web interface at http://[raspberry IP]:8888/gui. This interface allows you to create new “Shares” or connect a local folder to and existing one.