Wine allows OS X users to run Windows applications.
Note: this listing is for the official release of Wine, which only provides source code. If you want a version of Wine that is packaged specifically for OS X, then use Winebottler, available here.
Wine (originally an acronym for "Wine Is Not an Emulator") is a compatibility layer capable of running Windows applications on several POSIX-compliant operating systems, such as Linux, OS X, and BSD. Instead of simulating internal Windows logic like a virtual machine or emulator, Wine translates Windows API calls into POSIX calls on-the-fly, eliminating the performance and memory penalties of other methods and allowing you to cleanly integrate Windows applications into your desktop.