Win4Lin
Win4Lin is a powerful software application for Linux which allows a user to run a copy of Microsoft Windows 95 or 98 applications on their desktop. The system works by acting in a similar way to a Virtual Machine, in that it provides an environment for the Windows operating system to run, and as such one must still have a legal copy of Windows to use with it.
The Win4Lin application in essence displays a window on your Linux desktop which contains the Windows desktop environment. As an alternative, the Windows environment can be run full screen, outside of your desktop environment to save on Desktop Real-Estate.
Win4Lin is designed with business users in mind, and as such neglects support for features such as DirectX, and MIDI, over support for office-style application compatibility.