[ where do I find a simple home temperature alarm to notify neighbors if heat goes off? Like turning on a light seen from outside?

Sensaphone is one method.
Another is A thermostat, relay, and a light, bell or buzzer. Intellisense makes a nice sensor, that will close when cold or heat is exceeded. You can also hook to a dialer so it will call several numbers(your cell, neighbors home phone, whoever, anywhere).
Remember, when temperature is reached, it will dial. May be better to be alerted to loss of power to heating system.
Another thought is have it dial or alert when temperature is exceeded, and not when heat is off.