Protocols

NIST Cerberus
An IPsec reference implementation for Linux.
http://www.antd.nist.gov/tools/cerberus/

OpenBSD VPN Mini-FAQ
IPSec VPN configuration information.
http://pintday.org/hack/docs/vpn-24-minifaq.shtml

Point to Point Protocol Extensions Working Group (PPPEXT )
An IETF group working on PPP's extensions.
http://www.ietf.org/html.charters/pppext-charter.html

Point to Point Tunneling Protocol
PPTP technical specification by 3Com.
http://infodeli.3com.com/infodeli/tools/remote/general/pptp/pptp.htm

PoPToP
PPTP server solution for Linux. Includes downloads, documentation and news.
http://www.poptop.org/

PPTP Client
A Linux, FreeBSD and NetBSD client for the Microsoft Point-to-Point Tunneling Protocol.
http://pptpclient.sourceforge.net/

PPTP Crack
Counterpane Systems has exposed insecurities in Microsoft's Point to Point Tunneling Protocol.
http://www.schneier.com/pptp.html

Stunnel
A GNU program allowing to encrypt arbitrary TCP connections inside Secure Sockets Layer (SSL).
http://www.stunnel.org/

VPN-Protocols
Technologies used for creating secure VPNs. Various standards and protocols listed with links to related RFCs.
http://www.vpnc.org/vpn-standards.html