Tunnelblick
A graphical user interface for OpenVPN on Mac OS X
...It comes as a ready-to-use application with all necessary binaries and drivers (including OpenVPN and tun/tap drivers). No additional installation is necessary -- just add your configuration and encryption information.
To use Tunnelblick you need access to a VPN server -- your computer is one end of the tunnel and the VPN server is the other end.