"Drive-By-Wire," or Electronic Throttle Control (ETC) is a system which severs the direct connection between the accelerator pedal and the throttle control in the engine.
Up until recently, most automobiles had a cable attached to the accelerator leading to the throttle which controlled how much of the gas/air mixture went into the cylinders. Nowadays, however, a lot of cars come with Drive-by-wire systems that send a signal to an electric motor that opens and closes the throttle. You can find this technology in such new cars as the Honda Fit and the BMW Mini Cooper.
This makes a lot of options such as Cruise-control and anti-lock braking easier for the manufacturer to install.
Drive-by-wire doesn't necessarily have to associate itself with just the throttle, however. In the near future, we'll be seeing cars with drive-by-wire steering, brakes, and many other systems that would normally be controlled by the driver.
One of the most prominent examples today is the Toyota Prius. It requires drive-by-wire systems for its Hybrid-Synergy Drive because of the complications of succesfully integrating electric motors and a gasoline engine to provide a smooth ride while returning high fuel mileage.
Some say that this technology is dangerous and destroys the connection between the driver and the auto. This is not entirely true, as many fail-safe devices have been installed to prevent tragedy from a simple breakdown.
The Honda Fit comes standard with a drive-by-wire system on the throttle.