Synqion (was TeamDrive) virtualizes folder and their documents for data exchange and collaboration among teams, individuals, and multiple devices using the Internet. Anyone with access to a WebDAV server (e.g., available with Box.net, free-of-charge) can use the free version of Synqion for an unlimited time without any restrictions. The software application guarantees a high degree of security and is awarded the Data Protection Seal of Approval by the Independent Center for Privacy Protection ULD in Germany. All data which leaves a computer is highly encrypted (256 bit) and is not accessible by any uninvited person. All data is available offline and always stays in sync.
A basic Synqion account is free. Personal ($110.85/year) and other versions are also available. For more details see this page.