WorldWide Telescope, an extraverted version of Google Earth, is essentially a map of the stars with markings and astronomer's expert explanations of what you're looking at. The data is provided from various sites like NASA and Wikipedia among others, and the images are compiled from the Hubble and the Cerro Tololo Observatory in Chile.
Much like the Google Earth and Google Sky applications, you do a simple download and you can start exploring right away. They'll be adding more features soon enough to bring it up to speed with Google Sky which is slightly ahead in innovation and you can bet the competition is good for both entities.
On a separate note, we are obsessed with this site which updates daily.
worldwidetelescope.org to download and read up
Microsoft Unleashes WorldWide Telescope
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