Boxes!

VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product 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) version 2.

Official description: VMware Workstation Pro Lets You Run Multiple Operating Systems as Virtual Machines (including Windows VMs) on a Single Windows or Linux PC. VMware Workstation Pro is the industry standard for running multiple operating systems as virtual machines (VMs) on a single Linux or Windows PC. IT professionals, developers and businesses who build, test or demo software for any device, platform or cloud rely on Workstation Pro.

VMware Workstation Player builds on the industry leading foundation of Workstation Pro, and delivers a streamlined user interface for creating and running operating systems and applications in a virtual machine.

Parallels Desktop gives Mac users the ability to run Windows, Linux or any other operating system and their critical applications at the same time as Mac OS X on any Intel-powered Mac Pro, iMac, Mac Mini, MacBook or MacBook Pro.

Qemu Manager is an easy to use management tool for the open-source QEMU emulator. It has a wealth of features for creating, managing and running multiple virtual machines. It also allows you to run virtual machines within the program itself, giving you quick access to all your running virtual machines.