Using a Virtual Appliance in OS X
A good selection of open source operating systems (mostly Linux-based, such as Ubuntu) can be installed in VirtualBox, allowing you plenty of choice between virtual appliances.
There are two ways to use open source operating systems on virtual appliances. The first is to download your own Linux distro and install this onto your virtual machine in VirtualBox.
The second is to find a selection of legal guest operating systems on the web which you can download and use with minimum time spent on the configuration.
One of the best resources for VirtualBox virtual machines is virtualboximages.com, where you will find a good selection of images that can be downloaded and added into VirtualBox without first setting up the virtual machine element.
Images downloaded from that site can be used for a variety of purposes, from trying out alternative operating systems and applications to use as virtual development devices.
Once you have downloaded one of these images, select File > Import Appliance… to begin the OVF file format import wizard which will assist you in importing and using the virtual appliance!
Following these steps should be enough to assist you in taking advantage of VirtualBox appliance images. If you have any problems or questions, let me know in the comments below.