The APRS protocol has been adapted and extended to support projects not directly related to its original purpose. Here are a few of the most notable ones projects.
FireNet is an Internet-based system using the APRS protocol and much of the same client software to provide fire fighting, earthquake, and weather information in much higher volume and detail than the traditional APRS system is capable of carrying.
D-STAR Position Reporting System (D-PRS) utilizes the low-speed data stream of the D-STAR digital voice mode to transmit GPS NMEA sentences.
PropNET uses the APRS protocol over AX.25 and PSK31 to study radio frequency propagation. PropNET 'probes' transmit position reports, along with information on transmitter power, elevation, and antenna gain, at various frequencies to allow monitoring stations to detect changes in propagation conditions.
PropNET Homepage If the band is open and no one is active, does anybody hear it?