The Server & Tools Division that sells Windows Server, IIS, SQL Server,Lync Exchange, Visual Studio
But they don't make money off IIS
Visual Studio is largely given away in Express versions and the (.NET) developer market is relatively is relatively small compared to 'servers' in general.
Windows Server is... Windows, and while strong right now, probably is less important in a cloud based future. For basic File/Print servers, Linux is already a better choice in most cases. I have fairly solid Windows Server 2008R2 instances running apps and have no inclination to update them.
SQL Server has strong competition from Oracle at one end and MySQL etc at the other. Again the give away low end versions of SQLServer.
I suspect therefore the key revenue generator there is Exchange. There is no real effective open source or closed source direct competitor to Exchange, but currently there is no Outlook for Android/iOS, so customers are probably increasingly using Gmail (or IMAP to Exchange?) from mobile devices which weakens the propriety Exchange/Outlook functionality, and turns it into an open standards (IMAP/POP/SMTP) mail server.
So sure, they make a lot of money and that is not going to change overnight, but I can't see anything there being the next big thing.
Lync/Yammer/Skype might have some potential, but they currently give away Skype for free so can't see it.