MOUNTAIN VIEW, Calif.  GPS navigation devices were the latest must-have tech toys just two years ago, and shares of device makers like Garmin and TomTom were soaring.

That didn’t last long. In a turnabout that has been remarkably swift even for the fast-moving technology business, those companies have suffered as competition has pulled down prices  and as more people have turned to their cellphones for directions.

In the latest blow to the business, Google announced a free navigation service for mobile phones on Wednesday that will offer turn-by-turn directions, live traffic updates and the ability to recognize voice commands. The service will initially be available on only one phone, the new Motorola Droid, but will be expanded to more phones soon.

In a briefing on Tuesday in advance of its announcement, Google said that the service might be supported by advertisements in the future. That would make driving directions the latest form of information to shift from being a paid service to one that is ad-supported.