Waze
Waze is a mobile app that lets drivers make use of social knowledge to get better information about traffic and driving conditions.
Waze automatically tracks where you're driving via GPS, and if you're going slow, it marks your map with a red line that all local users can see as an indicator of traffic. Waze also lets users map unusual events -- such as accidents or roadwork, and atach pictures. Finally, Waze lets users mark where maps are inaccurate, and enthusiasts can go on to the web app and fix the map.