Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...
GoodSync - GoodSync provides highly reliable file backup and synchronization for both individuals and businesses.
Dropbox - Online Sync and File Sharing
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.
Microsoft OneDrive - Secure access, sharing & file storage
Google Drive - Access and sync your files anywhere