FreeS/WAN: www.freeswan.org

OpenS/WAN: www.openswan.org

Stunnel: www.stunnel.org

OpenVPN: openvpn.sourceforge.net

PoPToP PPTP Server: www.poptop.org

Linux PPTP Client: pptpclient.sourceforge.net

Analysis of Microsoft PPTP Version 2 by Bruce Schneier and Mudge: www.schneier.com/pptp.html

Linux's Answer to MS-PPTP by Peter Gutmann: www.cs.auckland.ac.nz/~pgut001/pubs/linux_vpn.txt

tinc: www.tinc-vpn.org