Stormpulse

I saw this mentioned on TechCrunch and just messed around with it: Stormpulse

It's one of the sweeter mashups I've seen; it provides great visualization of hurricane and tropical storm activity and forecasts.  It has toggleable overlays to include clouds, projected storm routes by various computer models, etc.  It's akin to having one of the visualization systems that the major network forecasters use to show expected storm routes (as seen with Gustav).