Comment by mstaoru

Surprised nobody mentioned Intel vPro AMT so far. It is basically an always-on KVM that's part of CPU firmware, powered by an always-on 5V PSU rail. There is a scary amount of options, including unattended periodic (or alarm based) phone home, user acceptance or full user override, boot media spoofing, Serial over WiFi... All built-in into consumer(-ish) CPUs.