I guess you'd like my firewall. It's an old 486 machine with no hard drive. It runs entirely off a write protected floppy. Physically impossible to corrupt from the outside. Should someone get into it, all I have to do is reboot the firewall.
When setting it up, my buddy hung the IP on a hacker forum and let 'em work at it for a while. When they beat it, they told him how it was done and he worked to prevent that particular access.
It works pretty good. Especially when you consider that I'm not exactly hiding state secrets.