How to route internet traffic through a site-to-site OpenVPN tunnel in PfSense 2.1
How to use OpenVPN with Google Authenticator on Ubuntu 12.04 LTS server.
Set up OpenVPN on pfSense for Windows clients with certificates and user authentication via Active Directory RADIUS
This article shows you how to set up PfSense as an OpenVPN server that authenticates against Active Directory via Radius. Your domain users will be able to logon to the vpn using their domain credentials.