Netfilter : The current Linux kernel packet filter/firewall
Netfilter is a powerful packet filter implemented in the standard Linux kernel. The userspace iptables tool is used for configuration. It now supports packet filtering (stateless or stateful), all kinds of network address and port translation (NAT/NAPT), and multiple API layers for 3rd party extensions. It includes many different modules for handling unruly protocols such as FTP. For other UNIX platforms, see Openbsd PF (OpenBSD specific), or IP Filter. Many personal firewalls are available for Windows (Tiny,Zone Alarm, Norton, Kerio, …), though none made this list. Microsoft included a very basic firewall in Windows XP SP2, and will nag you incessantly until you install it.