iPhone/Android/Windows Mobile: Waze turns your GPS-enabled phone into a node in a network of real-time traffic reporters. As you use Waze, you build maps, get traffic data, and alert other commuters to events on the road.
Waze is focused on bringing you live traffic information and maps via crowd sourcing. Each phone with Waze installed generates map data and, through user-interaction, also provides information about construction, traffic jams, and other road-related events important to drivers.
Waze also provides turn-by-turn driving directions, which are adopted in real time based on traffic conditions and other information in the Waze network—it should be noted, however, that Waze currently doesn't function as a long range navigation tool but instead analyzes the areas you regularly drive in and suggests optimal routes.
Check out the demo from Waze to get a better feel for how the system works:
In addition to the application on your phone, there is also a dashboard system on the Waze web site. There you can flag areas of the map that need to be edited or repaired, make changes to the map to correct errors, and add additional information to the maps to help other commuters. Even if you don't use Waze on your phone, you can still visit the Waze site and view their live map of your area.
Waze is free and requires an iPhone, Android, Windows Mobile, or Symbian phone.