Another Arduino project I've been working on is a DTMF decoder used to control a relay board. Using a ham radio receiver, I can switch lights, radios, computers...anything...on or off from miles away. Here's the video:
Here's the wiring diagram. And here's the Arduino code.
I'm using a Sainsmart 4 relay board, although pretty much any relay board would work. You'll also need a MT8870 DTMF decoder - these run about $2 on ebay. And of course, you'll need an Arduino Uno. Again, check out ebay for these as well. The total cost here should be less than $12 and you've got a fully functioning radio controlled DTMF relay controller!