This howto describes the setup of the IPsec alternative OpenVPN, which is available for Win32, MacOS X and Unix-based Operating Systems. http://openvpn.net/howto.html
This document gives you the details how to setup a virtual private network between Linux/Openswan on one side and Microsoft Windows 2000 and XP on the other side. http://www.natecarlson.com/linux/ipsec-x509.php
This site gives valuable information about the Checkpoint VPN-1 product and its interoperability with other VPN implementations. http://www.fw-1.de/aerasec/index.html