I'm impressed with Google Earth. I hadn't totally got what was cool about it until I built my first Google Earth app earlier today. The documentation is pretty slight (it'd be nice if they simply broke down which elements are required and which ones are optional) but they have a good demo file you can simply copy.
After about an hour of hacking and I had a nice kml file with nearly 4200 points on it, direct from the MetaFilter user database. It's great that Google Earth launched the client with this robust API already in place.
The results are pretty cool. I've only offered lat/long values for a week or two, and already there are loads of people in far off places showing up. It's a good feeling to give the old globe a spin and see points all around, and to hear stories of people getting to know each other offline as a result.