After reading this blog by Alistair Grew regarding the Raspberry Pi in a backup scenario I got inspired to try out BitTorrent Sync. It is available at BitTorrent Labs for Windows, Mac, Linux (including NAS units) and Android.
Starting on my Mac, I installed and started the app. The first thing you need to do is to generate a long key (secret). But sharing this key between any of you computers (you or your friends have) a single folder will be kept in sync between all devices. You can sync different folders with different people – all simply by the sharing of the secret.
On the Raspberry Pi, the installation was very easy. Download the into a directory where you want to keep it. Un-zip the file. You also need to create the directory that will be kept in sync:
pi@raspberrypi ~/btsync $ tar zxvf btsync_arm.tar.gz
pi@raspberrypi ~/btsync $ mkdir syncfolder
cpi@raspberrypi ~/btsync $ d syncfolder
pi@raspberrypi ~/btsync $ ./btsync BitTorrent Sync forked to background. pid = 2508
The GUI is a webpage on the Pi on port 8888, so got to http://your-Rpi-ip:8888/gui
From here you may specify the folder to keep in sync and then things are done!