Dropbox - Online Sync and File Sharing
Syncthing - Syncthing replaces proprietary sync and cloud services with something open, trustworthy and...
ownCloud - ownCloud is an open source project enabling businesses to host their own cloud storage while maintaining regulatory and compliance needs.
Mega - Secure File Storage and collaboration
Microsoft OneDrive - Secure access, sharing & file storage
Google Drive - Access and sync your files anywhere