Openswan
Openswan is an IPsec implementation for Linux. It has support for most of the extensions (RFC + IETF drafts) related to IPsec, including IKEv2, X.509 Digital Certificates, NAT Traversal, and many others.
strongSwan
strongSwan is an Open Source IPsec-based VPN solution for Linux and other UNIX based operating systems implementing both the I ...
IPsec-Tools
IPsec-Tools is a port of KAME's IPsec utilities to the Linux-2.6 IPsec implementation. It supports NetBSD and FreeBSD as well. ...
Libreswan
ibreswan is a free software implementation of the most widely supported and standarized VPN protocol based on ("IPsec") and th ...