automatic packet reporting
I love learning about entire communications networks that I had no idea existed. Case in point: Nelson Minar blogs about APRS, the Automatic Packet Reporting System. "APRS lets you send short messages from a handheld radio to the Internet through the simple expedient of broadcast and repeat."
More info from Wikipedia: "In its simplest implementation, APRS is used to transmit real-time data, information and reports of the exact location of a person or object via a data signal sent over amateur radio frequencies. In addition to real-time position reporting capabilities using attached Global Positioning System receivers, APRS is also capable of transmitting a wide variety of data, including weather reports, short text messages, radio direction finding bearings, telemetry data, short e-mail messages (send only) and storm forecasts."
Read Nelson's post for more info; fascinating.