A little late chiming in here. XenServer could also be used but then the host would be dedicated to that role. One thing I really like it is it is able to copy the hardware ID from the box and pass it to the O/S. This allows you to virtualize OEM versions of Windows. I have never tried Virtual Box myself but I have always heard good things about it.
Those images are ostensibly for testing and research of IE browsers so they have a time limit and a watermarked desktop, but are otherwise complete and fully functional within VirtualBox on Linux, OS X, or Windows. The time limit is no big deal since you can just save your work and reinstall another one, then add your work to the new one. WARNING TO ALL: READ THE LICENSE AND INSTRUCTIONS!
Also it is now possible to install OS X Yosemite on an Intel-based virtual machine but you'll have to search for those instructions online.
^ I would recommend against using Modern.IE VMs as a long-term solution, or for other than "testing" purposes.
If you download one of these VMs, you are limited to a single-user environment on a single VM, with an unactivated copy of Windows that expires after 90 days. Additionally, you are only licensed to use it for "testing" purposes, and are specifically prohibited from using it in any commercial or "live" operating environment.
I am running Win 8.1 as a Hyper-V host with 6 vm clients. These are all legally license. I am have 2 x WHS 2011, Win 7, Win 10 and 2 Ubuntu servers. I have been running this configuration for many months with zero problems.
This is actually a very inexpensive way to get a fully licensed and powerful server. The hardware is far more expensive. ..ie 30tb is expensive
There are a couple of free virtualization products that you can use, such as Oracle VirtualBox or VMware Player. I've been using VMware Player for a long time and I can recommend it. It doesn't have as many features as VMware Workstation, but it's free and enables you to create virtual machines, so you can download an ISO file and just install it inside a virtual machine, just like you would do on a physical machine.