I prefer Linux to Windows for a handful of reasons. One of the obvious benefits is licensing, and with all of the virtualizing I do in production and testing, its nice to never have to think about licensing. Meanwhile, back in the real world, most of my clients use Windows based servers for their day to day tasks. The Windows OS license is generally licensed per install/server; the notable exception being Data Center Edition which is licensed per CPU.
With consolidation ratios ever increasing, we are always on the lookout for bottlenecks in systems. What about licensing? If you are running numerous Windows guests, are there ways to make smarter licensing moves? In a nutshell, yes.
Instead of trying to reinvent the wheel, I will steer you to a well written and very informative article detailing some of things you can do. This is well worth the read.
–himuraken