Well hotmail was not owned by Microsoft originally. The urban legend is that after they acquired it they had to keep running it on FreeBSD a lot longer than they wanted to because Windows was not up to the task at the time.
Edit: After a moment's thought... wasnt Hotmail originally running on Solaris or SunOS?
"Hotmail initially ran under Solaris for mail services and Apache on FreeBSD for web services, before being partly converted to Microsoft products, using Windows Services for UNIX in the migration path."
Edit: After a moment's thought... wasnt Hotmail originally running on Solaris or SunOS?