VirtualBox is a powerful cross-platform virtualization manager that will enable you to run a virtual operating system on your PC. You can run Windows and Linux on a Mac, for example, and switch back and forth between the different operating systems without rebooting.
It's a really neat solution for developers who want to test their software on multiple platforms, or pretty much anyone looking to experiment safely with a computing environment.
We used VirtualBox to run Chromium OS for an upcoming article and sometimes rely on it to try out new software. If something goes wrong, one can easily switch back to a previous snapshot and avoid a headache. Small business owners can utilize it to test software and network configurations without having to buy multiple computers and licenses.
It's surprisingly easy to use yet also very powerful. You can install and run as many virtual machines as you like -- the only limits are disk space and memory -- and it can run everywhere from small embedded systems or desktops all the way up to datacenter deployments and even Cloud environments.
VirtualBox is being actively developed with frequent releases and has an ever-growing list of features, supported guest operating systems and platforms it runs on. The most recent major release, 4.0, brought an improved user interface and added virtual hardware support. You can download VirtualBox for Windows, Mac OS X, Linux, and Solaris.