Tested out a free GPS iPhone app on the way home called Waze. It worked really well and you just can't beat the price. It's interesting in that it has a social aspect to it. If your average speed drops below a certain range, it asks you if there's an accident or some other element causing the congestion. This input automatically notifies all other Waze users in the area of the issue. There is also input for intersection cameras and speed traps where police are hiding out.
Check it:
Free GPS Navigation with Turn by Turn - Waze