Yes, Google Earth and Keyhole before it are wonderful things out of box, but if you are interested in getting your hands into the guts of the product take a look at NASA World Wind
As those of who are familiar with the the NationalMap know the front end can be a little slow at best. This GoogleMaps hack to include WMS shows that this front end could be greatly improved. Here is an article from Where 2.0 abouot integrating WMS into GoogleMaps … O’Reilly Radar > Where 2.0: First Steps At OGC Integration With Google Maps
Here is a blog dedicated to Google Maps “hacks”. We all love Google Maps and Google Earth
Another mapping page. It uses Java and is a little slow on open, but moves quickly once it is up. It is Vector based and is driver oriented. Enjoy.