We utilize OpenVPN as part of our ever mobile fleet of offices and sites.
OpenVPN allows us to quickly and reliably setup offices without the worry of static IPs, NAT or all the other problems that can be associated with IPSec or PPT firewalls.
OpenVPN is an SSL/TLS based VPN solution. Clients and Servers are authenticated with trusted certificate infrastructure.
Typically we implement OpenVPN using a micro-server at the site, this gives us the benefit of advanced configuration capabilities as well as being a low cost NAS.