Automatic Packet Reporting System (APRS) is a digital data communications system often used by amateur radio operators in the 2-Meter FM band. You may hear such data transmissions on 144.390 MHz FM and other frequencies.
DK7IN has published an interesting project which promises to let you transmit APRS data when connected to a licensed amateur FM transceiver and a GPS device. The project uses the PIC 16F84 and a handful of resistors, capacitors, diode and switching transistor. A zip file with source code in Basic and a hex file can be downloaded.
While this project was designed for use with an amateur radio transceiver, there is no technical reason why it couldn’t be used for data transmission on FRS, MURS or other licensee-free frequencies where data transmission is permitted.