The great thing about this project is that there aren't a lot of parts that you need to get started. You probably have most of the parts lying around your junk box and the rest you can order directly from SparkFun or your favorite supplier. I've made a list of parts available on my wiki
. SparkFun can supply the relay and PCB and your local hardware store will have your GFCI Outlet and electrical housing.
Now a brief note about safety. Every time you work with electrical lines you may be risking your life if you don't use the right precautions. In general you should always employ a certified electrician but you can do this project on your own if you're careful. Absolutely ensure the plug is not connected to a live electrical socket when working on the relay, the outlet, or the extension cord at any point. Also, it's probably good practice to enclose any wires before testing. With that you should probably do just fine.