Open Source

Linux FreeS/WAN Project
An opensource project and software package for creating IPsec VPNs to and from Linux machines.
http://www.freeswan.org/

Linux IP Tunnel Mini-HOWTO
Instructions on how to setup a IP tunnel under Linux
http://linas.org/linux/iptunnel.html

Linux VPN HOWTO
Setting up a VPN with ssh and ppp.
http://www.tldp.org/HOWTO/VPN-HOWTO/

Linux VPN Masquerade HOWTO
How to configure masquerading of IPsec and PPTP VPN traffic.
http://www.tldp.org/HOWTO/VPN-Masquerade-HOWTO.html

Linux-Sec: VPN Page
Collection of Linux VPN HowTos and free software.
http://www.linux-sec.net/VPN

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.
http://www.openbsd.org/

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

OpenVPN GUI
A Windows GUI for OpenVPN. Gives a systray icon with a menu to control OpenVPN.
http://openvpn.se/

PPTP_GUI Home
A graphical user interface to the pptp-client software for Linux and BSD.
http://pptp-gui.sourceforge.net

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

Secure Forwarding of Services with SSH
Contains ssh-ppp perl script to set up a VPN.
http://csociety.ecn.purdue.edu/~sigos/projects/ssh/forwarding/

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.
http://www.tinc-vpn.org/