Yes, such a beast does exist! And I am proud to say that Gaiocorp (ok, a bit of not-so grey advertising) has developed it and released is as an open source project in CodePlex (http://slmapviewer.codeplex.com). Lovingly called SLMapViewer, it is a reusable Silverlight map control for use with MapServer, the popular open source WebGIS. It was initially developed as the the mapping component for some of Gaiocorp’s products and was based on the MapFlash Viewer Framework’s concept developed by Paolo Corti. Since then it has changed almost beyond recognition, supporting a large number of ‘GIS-like’ functions, different ‘view modes’ to be used as either through a normal browser or in a street kiosk, and extendable helper classes so developers can implement their own business logic for layer display. SLMapviewer can be used as a standalone map viewer or be embedded it and thus spatially enable, ASP.NET or Silverlight applications.