Weather Underground has a Google Maps mashup on which anyone who has a weather station can upload their data and set it displayed on their website and on a Google Map. There are already hundreds, if not thousands of these stations plotted on the map (which defaults to the San Fransisco Bay area). Current and past temperatures, air pressure and humidity readings are available for each weather station. Weather stations must use compatible software (see details).

This is an incredible way of collecting data (provided everyone’s weather station is accurate and functioning). However, it quickly becomes apparent that there is almost too much data - the map icons fail to appear and the map becomes difficult to navigate.