OpenSwan
From DD-WRT Wiki
Someone needs to write what openswan is and why one would find it useful, along with a tutorial about how to install it on the DD-WRT firmware.
Here I've copy and pasted the discussion about OpenSwan from the main page to help get started:
We need to get some instructions going in here for how to load ipkg modules. I'm particularly inertested in getting OpenSwan (http://www.openswan.org) running on DD-WRT. Here's a link to some instructions for OpenWRT (http://openwrt.org) , but I'm not sure they work in DD-WRT:
http://lists.openswan.org/pipermail/dev/2004-May/000322.html
Numbski