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.
Google spends a lot of time debunking various scary thoughts about the privacy issues involved in this kind of thing, which makes it a little more worrisome. Google does indeed have access to ridiculous amounts of information, so they're taking measures which may or may not assuage your concern. Speed and location information is all anonymous, and Google tries to combine data from different sources together so it's tougher to tell exactly what information came from where. Still, we really like this idea—it's nice of Google to put their massive store of data to use for something simple and useful like avoiding traffic. [Google]