libnfnetlink is a low-level userspace library for nfnetlink-based communication between the kernel-side netfilter and the user-space world. It is therefore the fundamental layer for all other nfnetlink-enabled user-space programs interfacing with the netfilter subsystem of the Linux kernel.
links2world Firewall is an iptables script generator for Linux 2.4.x or newer kernels. The configuration file is very human readable and easy to understand and write. It does not matter if you have one, two, three, or twenty network interfaces; it is able to generate stateful iptables rule sets that control the packet flows between all the networks your machine is attached to.
lsfw (list firewall) helps network administrators deal with firewalling on a huge network. It lists the firewalls rules applied between two points on the network. It uses the configuration of the network equipment and builds a (light) model of the network described by the equipment. This allows probing for access-list matching all over the network, doing routing and firewalling.
m0n0wall is an all-in-one firewall software package that is based on FreeBSD. It is geared towards embedded PCs, but it also works on standard PCs. It includes an easy-to-use Web interface like commercial firewall boxes do. PHP is used instead of shell scripts, and the entire system configuration is stored in a single XML-formatted file. There is support for VPN, traffic shaping, captive portal, VLANs, and more.