This one just blew me away. Several cities in U.S. are using an AVL (Automatic Vehicle Location) kit provided by NextBus, a privately owned company based in Emeryville, CA. In fact, San Francisco is using NextBus for Muni's since 1999.
The kit includes a GPS receiver, a Cellular Digital Packet Data (CDPD) and the respective antennas. From a job posting at NextBus website:
"The NextBus system is a multi-tier system written primarily in Java and JSP's with a database back-end. NextBus configures its GIS data with a heavily customized version of ArcView. NextBus deploys hardware into the field that is programmed in C. The hardware communicates wirelessly to the NextBus servers."From NextBus' website you can check a bus route on a local map or through its interface to Google Maps. It will tell you exactly when the next bus is expected to arrive. You can receive this information on a mobile device (wireless phones and pda's). You can also set alerts that will tell when the next bus is supposed to arrive. Great tool.
Gregory Szorc's Blog (which first pointed me to NextBus) describes how to debug the NextBus' API.