After those two I'd count Lotus Notes as another reason many are strongly motivated to MS Windows. Is there a linux version of Lotus Notes compatible wih the MS-windows version?
Well, it's more Outlook-ish, but Ximian Evolution is very full-featured and nice. Contacts, calendar, etc.
BTW--OpenOffice and StarOffice pretty much have caught up to MS Office. I use OpenOffice at work, which is exclusively MS Office. No one knows the difference, and I've been able to open, read, and edit Word and Excel files (even with macros).
VMWare. The best of both worlds.