There are very few emulator programmes available for Windows, and even fewer that are free. If you want to instal a new OS, partitions must be created and so on – a lot of work to run the odd Linux programme.
Luckily, VirtualBox, a piece of software created by Oracle, comes to the rescue, offering the power and flexibility of more popular emulator apps (such as Parallels or VMWare Fusion) for free. As part of Oracle's enterprise suite, VirtualBox enjoys regular updates and bug fixes. Alongside this, VirtualBox supports a wide range of operating systems, including older versions of Windows.
Throughout my usage of VirtualBox, I have never experienced a crash of the emulated OS, even when running more intense programmes. Obviously this is somewhat hardware based, but Oracle's backing of VirtualBox pays dividends in performance,stability and continued updates to the software.
Being able to run a different OS from a free, multi-platform app is fantastic and, as such, VirtualBox more than justifies its place on this list. The software has been continuously updated throughout 2014, offering bug fixes and feature additions, and it continues to get better and better.