Many times when I’m moving a window it is to take a peak at something that lies behind it. There are tools I’ve seen in the past where anytime you start moving the window it will immediately become transparent so that you can see through to what is in the background, but it quickly becomes annoying when it happens every time you move a window.
Liquid Drag is a very lightweight app that tries to provide the best of both worlds. With Liquid Drag running you’ll want to move a window as you normally would with the left mouse button, and while dragging the window press the right mouse button to enable the transparency. This is an elegant solution to not always making windows transparent when dragging them, and at the same time making it easy to use.
The app is very light on resources (about 2MB of memory on my machine), and is worthwhile if this is a kind of feature you want. The one thing I should note is that there is no interface for enabling/disabling this, which means if you want to turn it off you’ll have to pull up the Task Manager, find the AppTransparency.exe process, and kill it.
Liquid Drag Homepage (Windows only; Freeware)