Custom GFCI Electrical Box
Pictured here is a custom made GFCI electrical box. GFCI stands for Ground Fault Circuit Interrupter. This device was fairly easy to make. I used a standard junction box with two sets of 3 prong outlets. The one on the left is a GFCI protected outlet the one on the right is a standard outlet wired downstream of the GFCI. All outlets wired downstream of a GFCI are also protected -- thus all 4 outlets are GFCI protected. Then I added a matching face plate and attached the electrical cord and added a standard 120 volt three prong plug. All these parts are available from home improvement stores.
This design is nice in that it is very portable, cheap to make, and does not require modifications to your house wiring.
A GFCI device works by monitoring the current load on the hot leg and the neutral leg of the circuit and if they are not equal the GFCI breaks the circuits.
Be sure to test your GFCI regularly.