FreeFileSync is an open source and free tool hosted on source forge.
It's very useful for those who have multiple machines,
I have got two laptops, two windows desktops and one Mac machine in office.
and all machines have one folder in common "Software"
I always want to have these folders in synch, If I download a new software on my machine
and save it to software folder, i want it to be available on machin2 to machine 5.
This software helps me in doing that. It's really great. No matter on which machine i downloaded.
I have a sync job created using this software which I have scheduled to run everyday, when I go for lunch.
Now software folder of my all 5 machines are in sync they contain the newest files.
I don't need to remember on which machine i downloaded a software, i can always search for it software folder of my machine.
You can also use it for keeping mirror copy of your one folder to other machine( or other folder on same machine) ...
You can also exclude files based on their extension, you can also exclude folders sub folders...
Usability point of view software is very easy to use there are very less options, yet it is highly customizable, User Interface is also quite good.
It also has a portable version.
Only shortcoming that I found and i hope will be fixed in future is fize size based exclusion.
Example: Ignore files greater than : 100 MB
To know more or download go to Product Home page
This is the best syn utility I have so far come across. It looks like my search ends here.