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Wine - Open Source Software for running Windows applications on other operating systems.
QEMU - QEMU (short for "Quick EMUlator") is a free and open-source hosted hypervisor that...
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.
VMware Workstation - VMware Workstation is a multiple operating system handler to easily evaluate the any other type of new operating systems.
CodeWeavers CrossOver - Don't buy a Windows license, don't reboot and don't use a Virtual Machine. Try a free trial of CrossOver to run your Windows software on Mac, Linux and Chrome OS.