Oracle VM Virtualbox is free, open-source software from the Oracle Corporation. The software allows you to create a virtual computer system on your hardware that can run a number of different operating systems. It's an excellent tool to use when you wish to emulate a customer's systems as part of the development process.
To install a virtual system you must download the latest version of the VirtualBox software from Oracle. There are binaries files of the software available for installation on Microsoft Windows, Apple OS X, Solaris, and Linux systems (Ubuntu, Debian, openSUSE, Fedora, Oracle Linux, and an EL5 build for other distributions).
Once the software is installed, you can build a system with a range of operating systems. Available distributions include Microsoft Windows (from Windows 3.1 onwards), Linux (many distributions), Solaris (from Solaris 10), BSD (FreeBSD, OpenBSD, NetBSD), IBM OS/2, and MAC OS X.
The actual build process is very straight-forward and highly configurable. For a detailed explaination of the process the VirtualBox online user manual is excellent. There are also many great videos on YouTube if you prefer this learning format.
While the VirtualBox installation will not have the same level of performance as a dedicated physical installation, I've found the performance and response to be adequate even on quite limited hardware.
You can find out more and download the software by visiting the Oracle VM VirtualBox website.