Notes: The MOSFET is of a P-Channel type, can be pretty much any P-Channel as long as the On-Resistance is low and it can sustain sufficient current. The NPN transistor can also be pretty much anything with sufficient hfe (>50), and can tolerate >100mA collector current spikes.
The purpose of this switch is to turn on moderate voltages and loads on the high side, so the ground remains common when off. All the commercial modules for Arduino type of development offer Low Side switching, and thus are unsuitable, as they float the ground and increase the wire count.
Here is a prototype board with two switch circuits:
