MirrorBackup.net - Mirror Backup protects from data loss by continuously monitoring files and folders, and backing them up to the cloud or a local drive when they change.
Duplicati - Free backup software to store backups online with strong encryption. Works with FTP, SSH, WebDAV, OneDrive, Amazon S3, Google Drive and many others.
AnyBackup - AnyBackup is designed to keep two pools of volumes in sync with one another.
rsync - rsync is a file transfer program for Unix systems. rsync uses the "rsync algorithm" which provides a very fast method for bringing remote files into sync.
@MAX SyncUp - An advanced tool for Windows to compare and synchronize files and secure data backup either locally or to FTP/SFTP, Google Drive, and more.
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.