We present ARWin, a single user 3D augmented reality desktop window manager, placing 3D user interfaces into a physical desktop workspace. We explain our design considerations and system architecture, exhibiting the ease with which such a system can be developed and used. We showcase a number of novel 3D applications, which take advantage of the environment to provide more powerful interactions and data visualizations. We elaborate on how to support 2D legacy application functionality. Finally, we discuss future possibilities for application and interaction development.