http://www.netheaven.com/TunnelTypes.html
http://vpn.shmoo.com/vpn/vpn-pptp.html
Olaf Titz from the CIPE
CIPE web page
http://www.quiltaholic.com/rickk/sslwrap/
OpenSSH web page
http://www.se.openbsd.org/faq/faq13.html#SeeAlso
http://lartc.org/howto/lartc.ipsec.html