I bet there would be a spike of interest in packaging stuff for cygwin, too.
Windows Server is unlikely to be free any time soon. Windows desktop might be, if they can plug/bundle some subscription models into it (perhaps office).
Even if Server was free (improbable like I said), that alone might not make it a competitive development or deployment environment. A lot of important work currently happening involves Open Source collaboration, and a free-as-in-beer ecosystem will not be able to take advantage of it. Earlier today, I was reading up on how AppFabric (scaling middleware) is evolving. Looks like it won't, and will soon join the other near-abandoned frameworks (Workflow Foundation, WPF, Silverlight) on the Windows stack. The future of scaling (on Windows) will have something to do with containers just like Linux or BSD. A Windows only approach to solving general industry problems is not going to cut it anymore.
In such a scenario, the primary development and deployment platforms supporting important industry trends will be free-as-in-speech Open Source platforms.
Add: Not saying MS is going anywhere. They have plenty of momentum.
Free for business would kill Microsoft, as much as they bet on their hardware locked in devices these days. They're not Apple just yet.
Everything else can be emulated away, but no accountant will ever work without MSExcel, no lawyer will work without MSWord, and Outlook is still the best integrated mail/calendar tool. Exchange and SQLServer are profitable, but they don't provide the same amount of lock-in and revenue that Office does.
However, this won't continue forever, especially with the move to mobile. I'd argue that giving away Windows Server would be a good move to try and secure the Office monopoly beyond the next five years.