OpenVPN is a robust, scalable and highly configurable VPN (Virtual Private
Network) daemon which can be used to securely link two or more private networks
using an encrypted tunnel over the internet. It can operate over UDP or TCP,
can use SSL or a pre-shared secret to authenticate peers, and in SSL mode, one
server can handle many clients.

WWW: http://openvpn.net/index.php/open-source.html