Ubuntu, OS X are all reasonable replacements. I'm sure there's an open source replacement for things like Exchange and Active Directory. After all Ad is based on OLAP.
http://www.smallbusinesscomputing.c...Source-Alternatives-to-Microsoft-Exchange.htm
http://www.osalt.com/active-directory
http://www.mysql.com/ --> Alternative to SQL Server
and so on.
2014 = The Year of Linux finally.
There are alternitives , but. Tons on " pros " and consumers have tons of money in windows software, big enterprise, millions invested.
I'm aware there are alternitives, but so far, linux isn't user freindly, osx is only on macs, neither of them give most people a reason to switch
Some people need windows, like me. The softwate we use, no nix version. And even if there was an osx version. Osx wouldn't be able to do it.