The Tracker2 is a new product of the OpenTracker series from Argent Data Systems.
Several of our GM RAYNET members have tried these out as a beta-test kit, and can highly recommend it.
The Tracker2 can operate in the following modes:
as a standard Tracker, to send out the position of a mobile;
as a reverse tracker, to display the position of other mobiles on a GPS display;
to interface to a palmtop terminal, as a UI messaging TNC;
as a stand-alone digi-peater unit (with remote control over the air!);
as a KISS TNC.
Firmware upgrades may be downloaded into the unit (the latest version exploits the Fleet Management Interface in some of the new Garmin Sat-Nav products, which provides a complete tracking and messaging interface for use in the car).
See some examples of my Garmin Nuvi used in this mode.
It seems to be an ideal unit for RAYNET field operations, and is now available as a product for around $95, see the Argent web-site.
Have a look at this quick-start guide which I compiled.