Porting Kit - Install Windows Games in Mac OS X natively and easily and for free with Wineskin technology
Wine - Open Source Software for running Windows applications on other operating systems.
Vineyard - Windows programs on Linux, easily
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.
Lutris - Lutris is an open source gaming platform for GNU/Linux.
WineBottler - WineBottler is an App to manage and wrap your Windows apps into convenient OS X apps.