VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See "About VirtualBox" for an introduction.
Bitnami offers a lot of useful applications that can be run from the cloud (with a subscription) or on a VirtualBox on your computer for free. I especially like this site because it has virtualized version of Learning Management Systems (LMS) like Moodle and Canvas, and Content Management Systems (CMS) like Drupal and Joomla that you can set up to play with before deciding on which to use.