Firewalls are essential to every organistaion. Apart from the expensive commercial ones, there exist numerous open source counterparts. It is usually difficult to choose the best open source Firewall that suits your needs. So here is a list of the most popular and best Open Source Firewall alternatives.
PF (Packet Filter, also written pf) is a BSD licensed stateful packet filter, a central piece of software for firewalling. It is comparable to iptables, ipfw and ipfilter. PF is developed on OpenBSD, but has been ported to many other operating systems.
PeerGuardian 2 is Phoenix Labs premier IP blocker for Windows. PeerGuardian 2 integrates support for multiple lists, list editing, automatic updates, and blocking all of IPv4 (TCP, UDP, ICMP, etc), making it the safest and easiest way to protect your privacy on P2P.
pfSense is a free, open source customized distribution of FreeBSD tailored for use as a firewall and router. In addition to being a powerful, flexible firewalling and routing platform, it includes a long list of related features and a package system allowing further expandability without adding bloat and potential security vulnerabilities to the base distribution. pfSense is a popular project with more than 1 million downloads since its inception, and proven in countless installations ranging from small home networks protecting a PC and an Xbox to large corporations, universities and other organizations protecting thousands of network devices.
IPFilter is a software package that can be used to provide network address translation (NAT) or firewall services. To use, it can either be used as a loadable kernel module or incorporated into your UNIX kernel; use as a loadable kernel module where possible is highly recommended. Scripts are provided to install and patch system files, as required.