bochs!

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.

Seamlessly run your favorite Windows applications and devices, on any Intel-based Mac. Built from the ground up for the Mac, VMware Fusion is the easiest way to switch to Mac—letting you bring all of your Windows applications with you, making the most of your Windows software investment, while providing the perfect safety blanket for the switch.

Portable-VirtualBox is a free and open source software tool that lets you run any operating system from a usb stick without separate installation. It unpacks and adjusts the paths and configuration of VirtualBox so it will run portably. It will not support bridged networking through the virtualbox network adapter as it is not installed.

VMware vSphere Hypervisor ( based on ESXi ) is a free bare-metal hypervisor that virtualizes servers so you can consolidate your applications on less hardware.

XenServer is a turnkey open source virtualization solution that provides out-of-the box virtualization and cloud computing. XenServer includes the Xen Hypervisor, the enterprise ready XAPI toolstack and integrations for cloud, storage and networking solutions.

OpenVZ is container-based virtualization for Linux.

WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps. WineBottler helps you with running Windows programs on OS X.

MobaLiveCD is a freeware that will run your Linux LiveCD on Windows thanks to the excellent emulator called "Qemu".MobaLiveCD allows you to test your LiveCD with a single click : after downloading the ISO image file of your favorite LiveCD, you just have to start it in MobaLiveCD and here you are, without the need to burn a CD-Rom or to reboot your computer.

Boot Camp is a utility by that assists users in installing Windows XP, Windows Vista, Windows 7, or GNU/Linux operating systems on Macs. Boot Camp guides users through non-destructive re-partitioning (including resizing of an existing HFS+ partition, if necessary) of their hard disk drive and using the Mac OS X Leopard or Snow Leopard disc to install Windows drivers.