[ytjohn]

For home users, yes. But in the business world, it's the Microsoft Office Suite, especially Outlook + Exchange. Sure, MS Office has been ported to OSX, but it's more expensive to go with Apple Hardware than commodity PC + MS Windows.

If Microsoft flipped their lid and ported Office to run on common Linux distributions (Debian/Fedora), you'd see a lot more corporations switching.

Microsoft does have their Office365.com, which is a web-based version of Office. I haven't tried it myself, but from what I've heard, a lot of its advanced features rely on Silverlight and using browsers other than IE on Linux or Mac OSX puts you in a sort of basic mode. The advanced mode is very comparable to running the desktop version while the basic mode is akin to the Google Docs editor.