Amber sits in the tcpserver chain for qmail and implements an "amber list" for incoming mail. This neither accepts nor rejects connections, but implements a number of timing-related tricks to fake out spam and virus software that doesn't implement SMTP correctly. For example, it can defer requests from new IP addresses for a few minutes before it starts passing connections on to smtpd, pause and check for programs that send data before the HELO, and otherwise make things rough for bandit bulk mailers.
HTTPTunnel is a simple client/server application for creating an HTTP tunnel between two machines, optionally via a Web proxy. This tunnel can then be used to wrap arbitrary TCP socket traffic in HTTP, thus allowing communications even through a restrictive firewall that only allows outgoing HTTP connections.
Lokiwall is a firewall script for Linux. In addition to the standard function of easily limiting network traffic, it features dual routing (using two Internet connections simultaniously), connection fail-over, load balancing, traffic control, advanced masquerading, advanced DNAT, and advanced marking (to direct specific traffic to a certain interface). The advanced features require some kernel patches. The standard features will work on a default Linux system with iptables and iproute2.
Trustix™ Enterprise Firewall is a WYSIWYG firewall for iptables. It provides drag and drop security policy deployment and allows you to visualise DMZs, integrate branch offices with 3DES encrypted VPN tunnels, accelerate Internet access times with proxy caching server, and authenticate remote workers with PKI X.509 certificates. The unique GUI also allows you to manage traffic for all your zones (up to 24) as well as port forwarding and network address translation (NAT).