If you use Google Maps for mobile with GPS enabled phone, you can crowd source your live road congestion data on Google Maps traffic layers — just turn on Google Maps for mobile before starting your car to participate. When you choose to enable Google Maps with My Location, your phone sends anonymous bits of data back to Google describing how fast you're moving. Google continously combine your speed with the speed of other phones on the road, across thousands of phones moving around a city at any given time, to get a pretty good picture of live traffic conditions and send it back to you for free in the Google Maps traffic layers. If you'd like to stop your phone from sending anonymous location data back to Google, you can find opt-out instructions here.
To view live traffic, just zoom-in on the city, click "Traffic" button on Google Maps. The colors on the road correspond to the speed of traffic (relative to the speed limit of the road): green is free sailing, yellow is medium congestion, red is heavy congestion, and red/black is stop-and-go traffic.
More info: Mobile Blog