If you’re building any sort of web interface for geospatial data, take a peek at OpenLayers.
OpenLayers is being used as a front end for a lot of interesting datasets. My current favorite is a map interface to the CIA World Factbook (pictured above). You can click on any country in the world and quickly find out statistics like birth rate, population, and number of cell phones. Did you know that there are almost 10 times as many cell phones than land lines in Chad?