For people trying go above and beyond of Google Earth or Microsoft Virtual Earth and seriously building geoweb applications with complex geo data analysis and geoprocess, ESRI ArcWeb Service is the definite answer.
ESRI is building the next generation Flex based geoweb product to extend the existing offers. Go check it out. It has all kind of APIs you can play with.
Here is a video of demo application built on top of ArcWeb Service Flex API. We’ve seen this type of Flex book before. But it’s first time it got integrated into geo (GIS) concepts and data. Very visual and appealing.