Ipsec: Internet privacy and encryption information and protocols.
NIST Cerberus: An IPsec reference implementation for Linux.
OpenBSD VPN Mini-FAQ: IPSec VPN configuration information.
Point to Point Protocol Extensions Working Group (PPPEXT ): An IETF group working on PPP's extensions.
Point to Point Tunneling Protocol: PPTP technical specification by 3Com.
PoPToP: PPTP server solution for Linux. Includes downloads, documentation and news.
PPTP Client: A Linux, FreeBSD and NetBSD client for the Microsoft Point-to-Point Tunneling Protocol.
PPTP Crack: Counterpane Systems has exposed insecurities in Microsoft's Point to Point Tunneling Protocol.
Stunnel: A GNU program allowing to encrypt arbitrary TCP connections inside Secure Sockets Layer (SSL).
VPN-Protocols: Technologies used for creating secure VPNs. Various standards and protocols listed with links to related RFCs.