strongSwan

strongSwan is an Open Source IPsec-based VPN solution for Linux and other UNIX based operating systems implementing both the IKEv1 and IKEv2 key exchange protocols.

Free Open Source Mac OS X Linux Android

Libreswan

ibreswan is a free software implementation of the most widely supported and standarized VPN protocol based on ("IPsec") and the Internet Key Exchange...

Free Open Source Linux

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.

Free Open Source Linux BSD