The VPN Consortium, http://www.vpnc.org
The IETF, http://www.ietf.org
RFC-Editor's IETF RFCs, http://www.rfc-editor.org
The Linux Advanced Routing and Traffic Control HOWTO, http://lartc.org/howto/lartc.ipsec.html
Linux CryptoAPI, http://www.kerneli.org
Another IPSec tunnel implementation for Linux 2.4 from Tobias Ringström at http://ringstrom.mine.nu/ipsec_tunnel/
The IETF IPSec Charter, http://www.ietf.org/html.charters/ipsec-charter.html
Stunnel, http://www.stunnel.org
CIPE, http://sites.inka.de/sites/bigred/devel/cipe.html
IBM Redbook about VPNs, http://www.redbooks.ibm.com/redbooks/SG245234/nn4/SG245234_1.html
Linux 2.6 IPSec, http://www.ipsec-howto.org/
Internet IPSec Overlay Architecture X-Bone, http://www.isi.edu/xbone/
DHCPv4 Configuration of IPSec Tunnel Mode HOWTO, http://www.strongsec.com/freeswan/dhcprelay/ipsec-dhcp-howto.html
ftp://ftp.rfc-editor.org/in-notes/rfc3456.txt
The FreeBSD IPSec Mini-HOWTO, http://www.x-itec.de/projects/tuts/ipsec-howto.txt
The FreeBSD Handbook IPSec, http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/ipsec.html
The KAME Project, http://www.kame.net/
USAGI, http://www.linux-ipv6.org/
OpenBSD IPSec HOWTO, http://www.se.openbsd.org/faq/faq13.html
VPNLabs, http://www.vpnlabs.com/
Useful VPN Resources, http://vpn.shmoo.com/
The Keynote Trust Management Concept, http://www1.cs.columbia.edu/~angelos/keynote.html
IPSec road-warrior configuration, http://www.allard.nu/openbsd/index.html
Using IPSec on OpenBSD, http://www.se.openbsd.org/faq/faq13.html#