OpenVPN is a popular open source VPN protocol and it is cross platform. While OpenVPN runs on OS X there are no official packages for it. Most VPN providers offer their own OS X binaries that will usually support OpenVPN. A popular proprietary cross platform OpenVPN client is Viscosity. It is reasonably priced at $9 and there is a 30 day trial for Windows and OS X. But there are many reasons beyond cost, why a user may want to use an open source client.
A popular open source OpenVPN client for OS X is Tunnelblick. Tunnelblick is released under the GNU General Public License (GPL) and as such all the source code is available. I went ahead and downloaded the latest source repository to take a look. Technically, Tunnelblick is a Graphical User Interface (GUI) that interfaces with the OpenVPN code. The repository includes the OpenVPN source code as well as a few other open source projects it makes use of such as the popular OpenSSL project.
Ted Parvu
My hacking career began at age 12, when my father brought home a Sinclair ZX-81 computer kit and a lifelong fascination with computing was born. I became a privacy and security advocate after learning of the US Government's criminal investigation against Phil Zimmermann. Years later I found myself in Silicon Valley being paid to hack Phil's PGP code. I soon tired of the rat race and moved to the Northwoods of Minnesota where I have over 2 million acres of protected wilderness to roam. Here I indulge in my passions for wilderness and technology and as an advocate for computer security and privacy.