ActiveArmor is a hardware firewall designed by Nvidia that's built into their higher range nForce4 Ultra chipset motherboards.
It can reduce CPU load by making some simple layer 2 checks on incoming packets and discarding obviously malicious packets before they are sent to the CPU. It is an effective protection against some same subnet based attacks such as ARP poisoning, rogue DHCP servers and spoofed MAC addresses.
The ActiveArmor hardware firewall is nearly non-functional, with many unsolved bugs and potentially serious instability issues. Installing ActiveArmor can cause BSODs for users of certain software, especially peer-to-peer file sharing applications. Some programs, such as μTorrent, go so far as to have warning messages about using Nvidia's firewall in combination with their software. ActiveArmor also has a high probability of causing corruption of file downloads. Nvidia has been unable to solve these issues and points to hardware bugs within the chipset itself, problems which they are unable to work around.