I love this idea. Time to Test It: Free GPS Navigation with Turn by Turn - Waze |

Real-time maps and traffic information based on the wisdom of the crowd

watch the guided tour

From the site:

"What are waze drivers building?

Waze is a social mobile application providing free turn-by-turn navigation based on the live conditions of the road.

100% powered by users, the more you drive, the better it gets. Join the community of drivers in your area today!"