This article was recently linked to from Slashdot --
"Appearing late in the game, Microsoft's Virtual Earth, still in Beta, has some interesting features that may compete with Google Earth and World Wind. Utilizing NAVTEQ technology and USGS images, users can view road maps, aerial maps, and a bird's eye view of available locations."
Personally I had never heard of World Wind, I'll have the check it out.
Here is the link to the article --