Wine (Wine is not an emulator)

Wine is open source software that allows Microsoft Windows applications to be run on x86 processor-based Unix-like systems, including Linux, FreeBSD, and Solaris.

Wine does not act as a true emulator (the acronym itself stands for "Wine is not an emulator") but as a program layer that translates Windows application program interface (API) requests into equivalent POSIX code that Linux and other Unix-based operating systems use. Although commercial Windows emulators are available for Unix systems, these programs require users to have full Windows licenses and the operating system fully installed. Wine enables the user to run Windows applications without actually purchasing a Windows operating system end user license agreement (EULA).

The Wine developers say it is "not yet suitable for general use," but they also say that it will successfully run most popular Windows applications such as Word, Access, and Quicken.