Information on VPN gateways on Linux.
Windows 2k/XP contain L2TP IPsec VPN clients and may connect to a Linux VPN server running an implementation of IPsec, L2TP, and pppd. Below are links related to such a configuration.
Current Linux kernels (2.6.x) have a native IPsec implementation called 26sec. Prior to this IPsec was supported via FreeS/WAN which has been superceeded by OpenSWAN.