microcontroller low battery indicator
If you use a voltage divider with fairly high value resistors, e.g. two 499K in series and tap off the middle, your maximum voltage will be 4.5v and it will draw only 9 µA.

If even that much current were a concern, you could tie the bottom of the resistor divider into a open-drain output pin on the microcontroller and set it to ground only when you need to take a measurement.  In that case, you could get by with lower value resistors.

EDIT:

As Russel McMahon mentions, many micros won't allow the input of the ADC rise much above their supply voltage (VDD), which would happen when the ground was removed on the resistor divider.  So you would then need a high-side switch instead.