Telephone.... Ubiquitous!!!

The telephone is a sound device so ubiquitous that it often gets over looked. It's a valuable tool, yet because your micro-controller can communicate using a plain old telephone, either to exchange data, or to communicate using analog sound. The increasing ubiquity of digital networks has upstaged the use of sending analog tones to communicate over telephone lines, but telephones are still more common than data connections. Maybe you want people to be able to call from the street on their cell phones, and use their touch tone keypad to change the sequence of your christmas lights in your apartment. 

There seems to be plenty on the use of phones in the physical computing world, and is a great influence for my intial concept.