Poptop is an open source implementation of a PPTP server. Running under x86 or embedded Motorola ColdFire architectures Poptop provides full interoperability with the Microsoft PPTP VPN client.
Best open source pptp option I am aware of. pptp doesn't give the best encryption in terms of VPN options, but it is the most widely supported by different clients. If you have to support a diverse set of users, it's the way to go, at least as an option for some.