Google Maps is adding an incredibly useful tool to its repertoire, in the form of real-time traffic information. While currently available only in the U..S. and only for a select 30 cities, they will be expanding the service in the future. Most interestingly, the new service apparently gathers its information from multiple sources, acting as a conglomerate of data. The sources include things like road sensors and taxi fleets.
There are other services doing similar things, such as Yahoo Maps. Google's is not as expansive as Yahoo's at the moment. If you want to take a look for yourself, just visit Google Maps.