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CIPE - build encrypted UDP tunnels

ECliPt Secure Tunnel - another socket based encryption tool written in
Python.

HUT Mobile IP

Linux FreeS/WAN Project - implementation of IPSEC, ISAKMP/Oakley and DNSSEC

NRL IPsec + IPv6 - Naval Research Labs research project.

NIST Cerberus - an IPSec Reference Implementation for Linux.

Jonama - Jonama is a program that acts as a relay between a client over the
Net and your internal systems. It was developed with security in mind, ie.
it makes use of the SSL protocol to identify remotes and crypt channels and
uses Unix mechanisms like chroot and setuid to minimize rights and actions

PoPToP - a Linux PPTP server that works with NT Clients

PPTP-Linux - a PPTP client for Linux, haven't tried it myself

Slush - Slush is a project to create a simple and secure remote shell
application using SSL/TLS for communication security and X509 for
authentication. Slush is based on the excellent stunnel program. The
ultimate goal of Slush is to create a free and superior replacement for SSH
using open protocols and source code. Slush uses the SSLeay or OpenSSL
libraries for the TLS and X509 components.

Secure Sockets Agent - a system for securing the insecure or insufficiently
secure communication between the existing network applications. It provides
almost any client/server application with strong cryptographic security,
ensuring both integrity and confidentiality of the exchanged data as well as
authenticating both the client and the server. NON-OPEN SOURCE

Stunnel - designed to work as SSL encryption wrapper between remote client
and local (inetd-startable) or remote server. The concept is that having
non-SSL aware daemons running on your system you can easily setup to
communicate with clients over secure SSL channels. stunnel can be used to
add SSL functionality to commonly used inetd daemons like POP-2, POP-3 and
IMAP servers as well as standalone daemons like NNTP, SMTP and HTTP without
changes to the source code. Latest Version: 3.2 Last Update: May 24, 1999

tipxd - an IPX protocol tunneling daemon, running IPX over TCP/IP between
many hosts and unpacking them on the other side. This is especially useful
for people with masqueraded hosts who want to communicate IPX traffic to
others with masqueraded hosts.

Tunnel Vision - creates an encrypted network connection (virtual TCP/IP
network, or VPN), using very strong encryption (1024-bit RSA and 128-bit
Blowfish), and allows you to forward arbitrary TCP/IP packets between two or
more computers on the network.

Vpnd - a daemon which connects two networks on network level either via
TCP/IP or a (virtual) leased line attached to a serial interface. All data
transfered between the two networks are encrypted using the unpatented free
Blowfish encryption algorithm with a key length of up to 576 bits (may be
downgraded to a minimum of 0 bits to suit any legal restrictions).

VTun - "The easiest way to create Virtual Tunnels over TCP/IP networks with
traffic shaping, compression, and encryption."

x-kernelIPSEC - kernel space IPSEC implementation from U. Arizona