Well, there are multiple modules available on the market already at considerably low prices. For example, I've heard very good things about the X10 Remote Control module. I actually picked up a generic version of this at my local Hardware store.
But, if you insist on building one, there are many options to consider. You can use anything from IR emitting diodes and receivers, or possibly even photocells and a laser pointer.
However, I would personally utilize an open-source micro controller, being the Arduino whiz that I am.
You have to experiment around, and find what works for you.
If everyone else does all of the thinking for you, that takes the fun out of it.