Vitrite is a tiny Windows 2000/XP utility that allows you to manually adjust the level of transparency for almost any visible window. It works by utilizing a feature present in Windows (and all later incarnations) for variable window transparency.
When executed, Vitrite minimizes to the system tray in the lower right corner of the screen. If you wish to make a window transparent, you simply bring the window into focus (i.e. click on it) and press Ctrl Shift [any number on the top row on the keyboard]. There are 9 different levels of transparency: 1=90% transparency, 9=10% transparency, and everything in between.