You know when you are working on some grand Arduino implementation and you only have 2 available Gnd or Vcc pins and you need to then run a ground and power bus just to service the one extra module you are trying to add. It a pain in the arse for sure. But alas poor Yorick not everything is rotten in the state of Arduino, help is at hand. If you have a few vacant digital or analog pins, turn them into outputs and make them either VCC ot GND with a couple of lines of code and pull a pin either low or high. Code is better than hardware. Obviously current limits on pins apply and you cannot pull 100ma, but if the sensor or modules current limits are sparrow fart, then this is a nice way to solve a common problem.
A Ground Pin
digitalWrite(GND_PIN_#, LOW); pinMode(GND_PIN_#, OUTPUT);
A Vcc Pin
And here it is in action with a RTC.