netfilter!

Comodo Firewall Pro - Secure your system against internal attacks such as Trojan viruses / malicious software and external attacks by hackers. Safeguard your personal data through a simple user friendly single click interface offering full immunity to attack. Comodo Firewall Pro helps you connect in a secure way to the internet and global networks.

TinyWall is a free software to harden and control the advanced firewall built into Windows Vista, Windows 7 and Windows 8. TinyWall includes a combination of features that sets it apart from both commercial and freeware firewalls: TinyWall does not annoy you with popups at all, yet it is still extremely easy to add exceptions to your firewall rules; TinyWall does not require you to know about ports, protocols and application details; TinyWall prevents malicious programs from modifying the settings of Windows Firewall. »

Gufw is an easy, intuitive way to manage your Linux firewall. It supports common tasks such as allowing or blocking pre-configured, common p2p, or individual ports port(s), and many others! Gufw is powered by Uncomplicated Firewall (ufw).

IceFloor is the first OS X free graphic frontend for PF. Configure the PF firewall in a few clicks using a very simple interface and a step by step in-app configuration guide (Quick Start).

Murus is a frontend for the OS X PF network firewall. From basic filtering to advanced proactive options, Murus allows to easily configure a complex network setup dragging and dropping icons and moving sliders and selecting checkboxes.

Firestarter is an Open Source visual firewall program. The software aims to combine ease of use with powerful features, therefore serving both Linux desktop users and system administrators.

NuFW is a GPL extension to Netfilter (that is Linux firewall). It adds identity-based filtering, i.e. it is possible to create firewall rules dependent on the user.