OpenBSD - Secure Free Operating System
OpenBSD aims to produce a free, secure multi-platform operating system that integrates strong cryptography including a current and complete IPSec implementation.

Linux IP Tunnel Mini-HOWTO
Instructions on how to setup a IP tunnel under Linux

Linux FreeS/WAN Project
An opensource project and software package for creating IPsec VPNs to and from Linux machines.

Linux-Sec: VPN Page
Collection of Linux VPN HowTos and free software.

ENskip
A GPL'd port of Sun Microsystems Simple Key management for Internet Protocols

Secure Forwarding of Services with SSH
Contains ssh-ppp perl script to set up a VPN.

PPTP-linux client
Linux, FreeBSD and NetBSD client for the proprietary Microsoft Point-to-Point Tunneling Protocol, PPTP.

Tunnel Vision VPN
Creates an encrypted Virtual Private Network, or VPN, between two Tunnel Vision-capable sites on the Internet. Authentication is made as secure as possible by sending encrypted passwords first-time, and never thereafter.

OpenVPN
SSL/TLS based user-space VPN. Supports Linux, Solaris, OpenBSD, FreeBSD, NetBSD, Mac OS X, and Windows 2000/XP.

tinc
A self-contained VPN solution designed to connect multiple sites together in a secure way. Runs on Linux, FreeBSD, OpenBSD, NetBSD, MacOS/X, Solaris, Windows 2000 and XP.

Unofficial FreeS/WAN Site
Home of Super FreeS/WAN, ZeeS/WAN and FreeS/WAN + X.509 packages, as well as interoperability documentation and patches for additional features and broader compatibility.

PPTP GUI Home
A graphical user interface to the pptp-client software for Linux & BSD.

Linux VPN HOWTO
Setting up a VPN with ssh and ppp.

Linux VPN Masquerade HOWTO
How to configure masquerading of IPsec and PPTP VPN traffic.

OpenVPN GUI
A Windows GUI for OpenVPN. Gives a systray icon with a menu to control OpenVPN.