Here's an interesting application for commuters with GPS-enabled smartphones: Waze. Waze is a social traffic/mapping application. The idea is that Waze users leave the app running on their phone while they're driving around, and the application sends travel data back to a central server which aggregates that data and beams it back to all Waze clients. The result is that you get a real time look at traffic between you and your destination.
This isn't a new idea: some dedicated GPS navigation devices such as the Dash can do the same kind of thing. Waze puts the extra functionality of a smartphone to good use though, and offers features such as reporting specifics of a problem you've encountered on the road, or uploading images which will be shared with nearby Waze users. (Hmm, I wonder if you could use it to request help from a fellow driver?)
Waze's maps are user-generated; if you're the first Waze user to enter an area, the data your device sends back will actually be used to create a map of that area. So no taking shortcuts across cornfields while Waze is running, OK? Seriously, this illustrates both a strength and weakness of this kind of system: if you're around lots of Waze users you'll get great data, but if you're the only person in your neck of the woods using Waze, the software won't be of much use to you. But that's the nature of social software.
Waze is currently available on the iPhone and Android phones, with other versions "coming soon." Best of all? Waze is free.