OpenVPN is a full-featured SSL(Secure Sockets Layer) VPN(Virtual Private Network) which implements a secure network using the standard SSL/TLS protocol. It supports flexible client authentication methods based on certificates and/or username/password credentials and is supplied with utilities that greatly ease creation of the required PKI(Public Key Infrastructure) for issueing server and client certificates. The documentation is very good and everything needed to begin with is contained in the OpenVPN 2.0 HOWTO (another Howto here). OpenVPN creates the SSL tunnels over UDP or TCP, the latter even through an HTTP proxy if so desired. For Windows there is a pre-packaged installer which installs a TAP device and OpenVPN as a service. The GUI is conveniently located in the Windows taskbar and will create a connection to the OpenVPN server at the click of a button. The installer is available at openvpn.se and can be freely downloaded. Mac OS/X users also get a graphical user-interface: Tunnelblick is written in Cocoa, and the distribution contains all necessary binaries and drivers.