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.
RAS-SMS is a simple extension to Microsoft's Remote Access Service (RAS). RAS-SMS adds two factor authentication to standard VPN/PPTP clients on Windows, Mac-OS and Linux. RAS-SMS uses your mobile phone instead of smart cards, RSA keys or USB tokens.