Rice cookers do this, in a very simple way! They operate under four basic facts:

1. Assuming you've added the correct amount of water, rice is cooked when all the water has boiled away.

2. Water's temperature can't go over 100°C. After that, any additional energy goes toward boiling it away.

3. The temperature of cooked rice and air, without water, can go over 100°C.

4. Metals of different elements expand at different rates under different temperature conditions.

So the water gets up to temperature and begins to boil. As it boils away, it cooks the rice. Once it's all gone, the temperature of the cooked rice (and thus the cooker) begins to rise above 100°; when it does, one half of a strip of two metals touching the cooker expands further than the other, bending the strip, breaking a contact, and opening the switch, which turns off the heating element.

Expanding beyond this very simple mechanism is absolutely possible! But the more configurable you want the temperature to be, the more expensive it gets. I bet the simplest way to do this would be to have a few different little probes you can clip to the inside of the pan, one for each temperature you might want to keep a pan at. Inside each would be a bimetallic strip calibrated to that temperature.