Not only does Power Menu add the 'stay on top' option to all windows, but also it allows you to set the transparency of the window to make it see-through. This software also enables you to minimise an application to the Systray (the collection of icons near the clock on the Taskbar). Power Menu even lets you adjust an application's priority within windows, although I do not recommend setting this unless you are confident that you know what you are doing.
Power Menu works on all modern versions of Windows including XP and Vista. Once you have installed the software you can access the new features by pulling down the control menu of a window by clicking on its icon at the right hand of the title bar. The following images demonstrate Power Menu's excellent features:-
A semi-transparent Notepad.
Firefox minimised to the Tray. Click on the icon to restore it.