Netfilter and iptables are building blocks of a framework inside the Linux 2.4.x and 2.6.x kernel that enables packet filtering, network addresss [and port] translation (NA[P]T) and other packet mangling. This enable use as a firewall and router. http://www.netfilter.org
Macintosh based server-level and personal-user level software firewall, providing IP address based protection for TCP-based services. http://www.opendoor.com/doorstop/
ICSA certified appliance-based or software-based firewall. Simple, easy to configure and high-performance. Supports most applications and IPSec-based VPN's. http://www.gta.com/
All-in-one multi-platform Firewall with next-generation deep packet inspection technology. Includes IPSec VPN, Internet gateway (NAT), remote management, IDS, virus protection and PPPoE. Supports Windows, Linux and OS/2. Has a personal version. http://www.fx.dk/firewall/