FreeS/WAN: www.freeswan.org [1]
OpenS/WAN: www.openswan.org [2]
Stunnel: www.stunnel.org [3]
OpenVPN: openvpn.sourceforge.net [4]
PoPToP PPTP Server: www.poptop.org [5]
Linux PPTP Client: pptpclient.sourceforge.net [6]
Analysis of Microsoft PPTP Version 2 by Bruce Schneier and Mudge: www.schneier.com/pptp.html [7]
Linux's Answer to MS-PPTP by Peter Gutmann: www.cs.auckland.ac.nz/~pgut001/pubs/linux_vpn.txt [8]
tinc: www.tinc-vpn.org [9]
Links:
[1] http://www.freeswan.org
[2] http://www.openswan.org
[3] http://www.stunnel.org
[4] http://openvpn.sourceforge.net
[5] http://www.poptop.org
[6] http://pptpclient.sourceforge.net
[7] http://www.schneier.com/pptp.html
[8] http://www.cs.auckland.ac.nz/~pgut001/pubs/linux_vpn.txt
[9] http://www.tinc-vpn.org