selectable power voltage
You could use a digital potentiometer in the feedback path of a voltage regulator. 

From Analog Devices MT-091:


  Because they are 
  digitally controlled, digital pots can be used in active control applications, in addition to basic 
  trimming or calibration applications. For example, digital pots can be used in programmable 
  power supplies as shown in Figure 8A. Typical adjustable low dropout voltage regulators (such 
  as the anyCAP series) have a FB pin, where applying a resistor divider yields a variable output 
  voltage. As shown, R1 and R2 are the feedback and input resistors, respectively. The FB circuit 
  has an internal non-inverting amplifier which gains up a 1.2-V bandgap reference to the desired 
  output voltage.