ArcGIS Explorer is described by ESRI as being a “lightweight client for ArcServer”. Interestingly, ArcGIS Explorer is not actually tied to any other ESRI products. It and an SDK to develop customized tasks are free to download, and (as far as I can tell) free to deploy.
What this means is that ArcGIS Explorer can be used as an interface for all sorts of geospatial applications in a manner similar to Google Earth, Virtual Earth, and WorldWind using free and open source geospatial software components. Any processing logic that can be connected to any of these earth browsers can similarly be connected to ArcGIS Explorer. The current strength of ArcGIS Explorer in this category is the ability to customize the interface to hold any new toolsets.
I am really hoping to be able to work on some targeted geospatial applications that can very cheaply take advantage to this visualization platform.