Well the server is one which comes with IPFire. So that would be opensource.
http://wiki.ipfire.org/en/configuration/services/openvpn
With IPFire, all the server side certificates are configured using the web-panel.
The client is the one I got here.
http://openvpn.net/index.php/access-server/download-openvpn-as-sw/357.html
I trust they are compatible?