Software Developers
If you’re a Software Developer and you’re not leveraging virtualization then you’re pretty much nuts. By leveraging virtualization you can completely eliminate the cumbersome need for multiple pieces of hardware running separate configurations based on your needs at the moment. Instead, with virtualization, you can run multiple configurations or multiple branches of code on one machine with different virtualized instances with different operating systems or just different versions of your software. Never has testing and debugging your code been so easy!

QA Engineers
Quality Assurance Engineers not using virtualization should find a new job. Period.

Preservationists
Virtualization allows you to create easy backups of your entire virtual instance because your entire virtual machine is just a file. How much easier does preserving your precious data get than that? Go one step further and use some cloud computing to place your backup on your cloud server.

Environmentalists
With multiple machines comes lots of material that in turn came from nature. Not to mention all the electricity that goes into powering those things and, of course, if you are a real pro you’ll be using water cooling systems and especially in California water is a precious resource. Virtualization is one of the greenest things you can do by putting all of those machines in one place.

Educators
Kids these days are very tech savvy, I recall what we were doing to our school systems when I was younger and it wasn’t pretty. But you don’t have to worry about students saving work to a machine, changing settings, etc, when reverting everything back is as simple as installing a new instance of the virtual machine.

IT Guys
“I didn’t do anything” is that they all say when they completely wreck their work computer, but you can I well know that it’s almost ALWAYS operator error. So when they operator error their computer into oblivion just replace it with the virtual instance you have that you know works perfectly with everything they “should” have installed on their work computer.