Open source has grabbed a big part of the server and app dev market. Apple has redefined the mobile device market and rendered Windows Mobile devices beyond passé. Firefox has blunted Internet Explorer’s dominance, reversing the ActiveX hegemony for interactive Web apps, and removing one more barrier for widespread Macintosh usage. Vista is a dud — “a piece of junk,” one Gartner analyst called it yesterday in an interview. This week, it got worse. Google is now challenging the Office cash cow and Microsoft’s cautious steps into on-demand services through its new offering to let software vendors build Google-based apps. Salesforce.com has been remaking itself into a business app development and provisioning platform, and is likely to link up with Google next week on a joint offering.Almost everywhere you turn, Microsoft is challenged. (Exceptions: Its SharePoint collaboration platform is well liked and the fastest-growing product in Microsoft’s history, noted Gartner analyst Neil MacDonald, which could hep reinvigorate Office’s fortunes against Google’s incursions. Its Windows Server 2008 is a good OS that will serve businesses well. And its app dev tools dominate in the Microsoft-centric world of .Net.) Technology Industry