Cross platform file synchronisation programs I use

FreeFileSync

FreeFileSync is cross platform, open source program available in multiple languages that allows person to synchronise files and folders. 

I use it together with file sharing (described in this post) to synchronise files between my Linux laptop and Windows PC. 

 It detects all the folders and files (even with complex directory structure with multiple folders etc.) and scans both ends for changes in size, date and even contents.

There are many options available in it such as various synchronisation types (from left to right, from right to left, both sides or custom set), filters... and tonnes of other things to fiddle with. 

If you are person who does not have time to fiddle though - no worries, the program will synchronise your files perfectly on default settings, all you need to do is specify two places that will be synchronised. 

Program will compare them and produce a list of files showing which ones are going where. 

Synchronisation itself is hassle free - you can start it and then get on with other things, however if you like shiny things and nice looking graphs this this program will not disappoint you, it displays the speed, rate and whatnot during the synchronisation. 

On top of things, this program is also very fast - I was usually able to synchronise files with maximum speed (around 2-4Mb) between PC and Laptop on my home network, I was also able to synchronise files remotely (by using VPN) whenever I needed to access/save them from school.

MegaSync

MegaSync program is available on Windows, Linux and OSX which makes it ideal solution to sync files between very broad range of devices. 
The program itself is minimalistic but efficient, it has all the necessary options (such as proxy settings, bandwidth limiting and file exclusions), has decent GUI and is (partly as you can see on picture) available in multiple languages. 
The advantage it has over the other file hosting services is linked in to Mega file hosting - it offers great security features which (knowing background of Kim Dotcom and his relations with USA) most likely keep us away from all kinds of information spies and thieves and keep our data private.