Syncthing

Syncthing is still in development, although a large number of features have already been implemented:

Web GUI. Configure and monitor Syncthing via a responsive and powerful interface accessible via your browser.

Portable. Works on Mac OS X, Windows, Linux, FreeBSD and Solaris. Run it on your desktop computers and synchronize them with your server for backup.

Simple. Syncthing doesn't need IP addresses or advanced configuration: it just works, over LAN and over the Internet. Every machine is identified by an ID. Just give your ID to you friends, share a folder and watch: UPnP will do if you don't want to port forward or you don't know how.

Powerful. Synchronize as many folders as you need with different people.

http://syncthing.net

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Install Syncthing on Mint, Ubuntu...

$ sudo add-apt-repository ppa:ytvwld/syncthing
$ sudo apt-get update
$ sudo apt-get install syncthing

Install Syncthing GTK

$ sudo add-apt-repository ppa:nilarimogard/webupd8
$ sudo apt-get update
$ sudo apt-get install syncthing-gtk