OpenVPN is easy to configure on the newer DD-WRT firmwares.

On the DD-WRT revision 18946M, there is a Netmask bug (refuse to accept new Netmask), enter the Netmask in Router (TUN) Server Mode before switching to Bridge (TAP) Server Mode.

Use this OpenVPN guide to generate the required certificates: http://openvpn.net/index.php/open-source/documentation/howto.html#pki

Notice that I’m using TCP port 443, that allows me to bypass some firewalls 😉