The first indication that the North Koreans may have tested a nuclear device came from the the seismic data coming from the USGS. I thought that was kind of cool, and I had learned a few things about parsing XML feeds and the like, so I decided to try to make a little gadget which presented the latest earthquake information from the USGS plotted over a Yahoo! Map of North Korea.
Because there is little seismic activity in North Korea, and I needed some test data, I presented a similar feed for California. Try clicking on some of the markers to see information about seismic events (presumably non nuclear).