WinDataReflector is a lightweight file synchronization and backup tool that wraps up fast file comparison and transfer algorithms into a simple and clear interface that offers you visual comparison of files and folders before synchronizing and backing them up and support for two way synchronization with fully customizable rules. It is a freeware file backup application that's straightforward.
Here are some screenshots to illustrate the features and user interface of this application:
Syncthing
IDrive Classic
BUtil
FBackup
GoodSync