Dropbox - Online Sync and File Sharing

SpiderOak - SpiderOak makes it possible for you to privately store, sync, share & access your data from everywhere.

ownCloud - ownCloud is an open source project enabling businesses to host their own cloud storage while maintaining regulatory and compliance needs.

Box - Box offers secure content management and collaboration for individuals, teams and businesses, enabling secure file sharing and access to your files online.

Microsoft OneDrive - Secure access, sharing & file storage

Google Drive - Access and sync your files anywhere