When used on phones with GPS (including the Pre and MyTouch 3G, though not the iPhone), Google Maps crowdsources huge batches of data on how fast you're travelling on a particular street, thus measuring traffic for fellow drivers.
It's pretty cool, especially since it requires "just about no effort" on our part. All we have to do is load Google Maps before starting a drive, and it'll measure everything automatically. That data is then collected and mixed with other similar data until a clear view of traffic patterns emerges, which is available for all to see. It's not a new idea, but Google Maps is so widespread that it has access to tons more data than the similar Dash GPS.