The Wall Street Journal reported recently on Inrix, Inc., a Kirkland, Washington-based Microsoft spinoff that tracks speeds on 100,000 miles of US highway using data from GPS-enabled fleet vehicles, toll booths, road sensors...and citizens' mobile devices. The data is sold to a variety of companies (including MapQuest, Dash, and Garmin), which use it to provide traffic monitoring services to consumers.
Dash has perhaps the most interesting proposition. You buy their two-way navigation device and pay a monthly fee. The device reports your location and speed to Dash, and the locations and speeds of your fellow customers are aggregated and fed back to you as up-to-the-minute traffic reports.
This is crowd-sourcing writ large. It's also a perfect illustration of the network effect: that some systems become better as participation rises (highways themselves, ironically, not being an example). In 1878, Alexander Graham Bell gave Queen Victoria a telephone. It was suitable for the monarch in that it was very, very rare--as gifts to monarchs should be. Of course, its very rarity made it worthless. Only when every commoner had one was her telephone's full value realized (sometimes a commoner had a tricky question about table manners).
Traffic is certainly important but there are other situations in which you want to avoid crowds. Restaurants and museums are more attractive when lightly populated, for example, as are (up to a point) outdoor festivals and street fairs. Dash's service could easily expand to provide this kind of pedestrian traffic report.
In any case. Using people as a mass collection of mobile GPS sensors is a clever and powerful thing to do. I'm not participating yet, but I undoubtedly will be. And I look forward to the warm and fuzzy sense of belonging that comes from joining a Borg.