When I need to create a geospatial visualization, Virtual Earth is my default answer. When talking to someone who's been in this space for a while, he mentioned the MapPoint Web Service. I initially assumed this was a legacy offering that Virtual Earth replaced. Apparently not. Tatham Oddie has a very nice high level comparison to at least help you determine which makes the most sense in a given situation.
||MapPoint Web Service
||Road, Aerial, Birds eye
||Over 30 different styles (optimised schemes for night viewing, etc) however no aerial imagery
||JS control (best for embedding in web pages)
||SOAP web service (usable anywhere)
||Drag and drop positioning, scroll wheel support, interactive pushpins, AJAX based.
||Roll your own (it returns an image and you have to work out what to do with it).
||You create them all yourself on the fly using API calls – any clustering / filtering optimizations have to be done manually.
||Can upload pushpin sets to their databases and they will handle plotting / clustering and filtering.
||Specify a start point and an end point and they’ll give you a route in text. End of story.
||Specify the waypoints, preferred road styles (back roads, highways, toll roads, non-toll roads) and it will return a machine readable result set.
||Free (commercial use has some minor restrictions)
|SDK documentation and support
||Basic MSDN docs, active community (www.viavirtualearth.com)
||Plenty of MSDN docs and articles, including VS.NET integrated help and plenty of websites (www.mp2kmag.com)