Just a collection of pages we found on this topic, in no particular order:

How to Setup a VPN (PPTP) Server on Debian Linux (How-To Geek)
Configure a PPTP VPN Server on Ubuntu Linux (Networking Howtos)
Setting up a PPTP VPN Server on Debian/Ubuntu (Jesin’s Blog)
Steps to setup VPN in Linux using Graphical User Interface (Mr. VPN)
How to setup PPTP VPN on Linux (How To Install VPN)
Set-up PPTP virtual private network (VPN) server on Ubuntu Linux (Open Source Hacker)