Plus Zentyal provides a pretty damn good drop in for Active Directory so unless there's some odd AD specific feature you need Zentyal provides file sharing, groups, auth all for free.
I've taken over a client with it and they don't know or care about the difference except for the fact they don't have to pay for it.
Plus ESXi is free unless you need some major high level stuff.
Also more and more applications that are server based are moving to the cloud so you need less and less Windows servers.
Then there's MSSQL on Ubuntu which is free. 😃