OpenVPN is an Open Source VPN server and client that is supported on a variety of platforms, including pfSense. It can be used for Site-to-Site or Remote Access VPN configurations.
OpenVPN can work with shared keys or with a PKI setup for SSL/TLS. Remote Access VPNs may be authenticated locally, or using an external authentication source such as RADIUS or LDAP.
There are many possible ways to configure OpenVPN in pfSense. For those covered here on the GUI, see Category:OpenVPN.
Some common uses include: