PPTP

PPTP is a Point to Point Tunnelling Protocol originally developed by the PPTP Forum and used extensively by Microsoft as an easy VPN solution. It is installed in most Win98 or higher pcs.
It offers an encrypted login and a GRE based tunnel inside a TCP connection. It doesn't have a NAT Traversal mechanism (adjusting checksums or a 2nd tunnel), so firewalls must do this themselves.

A Linux server version is maintained at poptop.org
A Linux client version is maintained at pptpclient.sourceforge.net

It requires building custom kernel modules from your existing kernel sources, and can have quite a few surprises getting it built correctly. But once running, with the companion bcrelay programme and the latest ppp package, works very well.