Some of the AVR microprocessors have an on-die temperature sensor and bandgap reference, including the 75-cent-in-quantity 8 pin ATTiny45. By connecting up the ADC registers the right way it can measure its own supply voltage and use that to help calibrate the temperature sensor; even without more sophisticated calibration temperature measurements seem accurate to within a couple degrees C, and Vcc measurements to within a couple mV. All without any external components
Here's a '25 logging its own Vcc and temperature to a PC over a software serial link at 9600 baud. Measured Vcc on a calibrated DMM is 5.15 volts, its readings are bouncing around between about 5.14 and 5.18:
The temperature rise is me pointing a heat gun on low setting at the top of the chip from about 1.5 feet away.
You get serial output, i2c interface, and microprocessor for "free", too!