Controlling various devices remotely from a mobile phone or other Bluetooth-equipped device is fun and can be convenient when the project is developed into a final product. One example of this is the "Blu-box" by Kyle Brinkerhoff, who has documented a Bluetooth-connected Arduino-compatible device that can control relays remotely.
Once the device has been constructed, you simply need a mobile app which can send serial text commands via Bluetooth which can be easily interpreted by the Arduino sketch running on the controller. Doing so is even easier with Android devices thanks to the MIT App Inventor.
Along with some example Arduino sketches and the hardware schematic, Kyle has also published the files to create the enclosure shown above, so visit his Instructable for complete information. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're not inclined to make your own PCBs, it's much easier to use dedicated relay control boards. Here at Freetronics we have the RELAY4: board, perfect for controlling up to four relays with Arduino (or any other microcontroller's) digital output pins:
Or if you'd like to control up to eight relays - or more at once with an Arduino Uno or compatible board - consider our RELAY8: board. Controlled via I2C, you can stack up to eight shields with the appropriate power supply to support 64 relays at once: