VirtualBox is an impressive, open-source, desktop virtualization software. Available for Linux, OS X and Windows, VirtualBox allows you to run a different operating system on your machine, within a virtual environment. So for example, you could have Linux running on your Windows machine.
When you get something for free, you expect it to be lacking in some way, compared to commercial products which tend to have more features or more support. VirtualBox is surprising in that it is gives far more than you would expect, and is a real contender in the virtualization arena. Some of the VirtualBox features include the ability to move VMs between host systems dynamically, branched snapshots, and 32 way virtual SMP support.
What started out as a small virtualization project for individual users, has developed into a competitor in the enterprise sector. It still has some issues to sort out to make it as intuitive as it´s competitors, but remains one of the most comprehensive free apps available. Enterprises may still prefer to go with a commercial solution, but VirtualBox is an excellent option for users who do not want to pay commercial fees.