Virtual Private Network Consortium 
VPNC is the international trade association for manufacturers in the VPN market.
http://www.vpnc.org/How Stuff Works: How Virtual Private Networks Work
Gives definition of VPN and an overview of general VPN topics, includes diagrams and an animation.
http://www.howstuffworks.com/vpn.htm
About.com VPN directory
A smattering of VPN resources including a glossary and a self-test.
http://compnetworking.about.com/cs/vpn/index.htm
Corecom VPN FAQ Page
Geared towards those new to VPN.
http://www.corecom.com/external/vpn/vpnfaq.htm
Find VPN
Directory of VPN providers, articles, and links.
http://www.findvpn.com
NW Fusion: Research: VPNs
A collection of VPN resources and articles.
http://www.nwfusion.com/topics/vpns.html
Tom Dunigan's Virtual Private Networks page
This page describes IP-based VPN technology over the Internet
http://www.csm.ornl.gov/~dunigan/vpn.html
VPN labs - research community
VPN labs is an open community for researching, reviewing, and discussing Virtual Private Networks.
http://www.vpnlabs.org/
