Deep Space 1's ion-propulsion engine. It uses very little propellant and that means it weighs less, so it can use a less expensive launch vehicle and ultimately go much faster than other spacecraft. It's going to be the technology of choice for most missions where you to have to make large changes in the spacecraft's speed, or if you want to go to any extreme place. It's certainly a more legitimate option now. You couldn't ask for a better test.
Ion rockets are much too tame.
We'll never get up to light speed using ion propulsion. Rather, Grey sees a mix of missions, some powered by chemical fuel, others by ions. Flights to Mars , for example, could have people going on conventional rockets while cargo is relegated to the slower, ion-drive rockets. But you would only use it as long as you've got plenty of time to travel. Russia also has six communications satellites that use ion propulsion for station keeping.