As I write this, a line of thunderstorms is crossing Indiana, western Kentucky, Tennessee, and Mississippi. And there's a big storm in the Florida panhandle. How do I know? LightningMaps.org. It shows a live map with lightning strikes popping up a few seconds after they happen. The data comes from a network of monitoring stations and is visualized on top of a Google Maps view. As I watch, the delay is calculated at the top of the screen -- it's hovering between 4 and 5 seconds, so it may not be "real time," but it's close enough.
If you're curious, there's an extensive document (PDF) detailing how the system works, and how you can join the network.