FreeS/WAN, for Free Secure Wide Area Network, was the first project bringing IPSec to Linux.
The program was initially written to reach the goal of OpportunisticEncryption by first "securing 5% of the Internet traffic against passive wiretapping in 1996 [..] we can secure 20% the next year, against both active and passive attacks; and 80% the following year. Soon the whole Internet will be private and secure."
As the method they used was a general purpose IPSec daemon, it became far more popular for VPN tunnels across public networks such as the Internet.
Unfortunately, come 2004, the 5% goal wasn't even reached, and so the project stopped. Two forks called OpenSwan and StrongSwan took up the code from where it left off.
FreeS/WAN's own kernel IPsec implementation is called KLIPS, which was ported to the 2.6 kernel as part of the final 2.06 release. FreeS/WAN and derivatives can also use the new 26sec implementation.
Starting FreeS/WAN IPsec U2.01/K1.96... /usr/lib/ipsec/eroute: pfkey write failed, returning -1 with errno=22. Invalid argument, check kernel log messages for specifics.
pfkey has changed between 1.96 and 2.x - upgrade your kernel IPSEC support to 2.x or higher.
http://vpn.ebootis.de/ for Windows interconnection information.