Live Aircraft Tracking

There are two primary forms of these types of transponders, one basic which just transmits the aircraft altitude and unique identifier, and the other advanced which transmits location, altitude, heading, intent (lading, taking off, descending etc).

In the United States, most aircraft only use the basic version, so actual plotting all aircraft using these interception techniques is much more difficult. However, any international aircraft that fly in the US, or aircraft that are international capable, can be tracked.

There are out of the box commercially available receivers and software that you can use to get started... but again be warned that until US adoption becomes more widespread you'll only be able to track International flights.

For example: http://www.airnavsystems.com/RadarBox/index.html

Flightaware.com uses FAA feed (with a slight delay), but thus has full coverage of US airspace and a lot of other useful information (flight plans etc), but its' map sucks.