One of the best examples of sharing government data is with traffic information. The Utah Dept. of Transportation (UDOT) collects data along Utah's state freeways and highways. The real time data we're talking about comes from cameras, sensors, and reports. It is shared with the public on the State's Commuterlink website where users can see a mashup of all this information. But the data on Commuterlink goes well beyond UDOT. Traffic incidents are reported by the Utah Highway Patrol and other law enforcement and then made available through an XML feed to UDOT and others. Incidents are combined as another layer of the Google map mashup. National Weather Service data also impacts traffic so it is included in the site and users can get NWS data for their individual communities.
The Commuterlink app is multimodal so the same data is presented to mobile users through a different interface.
We could stop there, but the data becomes even more effective when it is publicly available. Take a look at these sites. They all use the same real-time UDOT / Public Safety data, but present it in different ways and with different functionality: