The WhereAVR is a small, lightweight, low-power, and low-cost APRS tracker much like the commercially available TinyTrak and the open-source OpenTracker. What sets it apart is a full complement of analog and digital I/O, as well as the ability to decode ax.25 packets. This allows for the reception of remote commands without the need for a "real" TNC. It is designed to hook directly to the speaker and microphone jacks of a handheld radio.
More information on WhereAVR can be had at its home page, http://www.knology.net/~gdion/whereavr.html.