WorldWide Telescope could be regarded as Microsoft's direct competitor to Google Sky, but I wouldn't agree with that.
WWT does contain space imagery that...
WinStars is a handy, free program only available for Windows, belonging to the category Science & education software with subcategory Astronomy.
Telescope 45x Zoom Camera Photo And Video is a free program for Android, that makes part of the category 'Photography'.
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Updated links to AltaVista, Ask Jeeves, MSDN, and Looksmart
SaturnMoons is a full version app for iPhone, that makes part of the category 'Education & Reference'.
Telescope Zoomer is a free software for Android, that belongs to the category 'Utilities & Tools'.
Star Adventurer Mini Console is a free app for Android, belonging to the category 'Photography'.