APRS: Automatic Packet/Position Reporting System
Some Ham Operators choose to "beacon" their current location, from their vehicle, bicycle, or even their backpack or purse, using the APRS System.
In North America, the most commonly-used frequency is 144.390 MHz Simplex.
An APRS beacon collects GPS information from the GPS Satellite Network, and broadcasts the Ham's Position, Speed, Bearing, and Altitude, to an "iGate" gateway (listening station), which in turn forwards the data over the Internet to the APRS Server System, where it can be used in many ways.
There are many "works out of the box" solutions, including Byonics.com; I use 2 of their "MT-AIO" units, which can be carried easily in a back-pack, or installed in a vehicle; they will run off of 8 AA batteries, or from external 12-volts DC.
But, you ask: How can I "see" where these Hams are at the moment?
This site works really well: APRS.fi
Here's another: FindU.com
A 3-D view of APRS! K6IB.com