By the launch of Windows 7 the level of transparency has changed in all apps,Now almost all apps comes with aero support.
Some time when you are working on two window you need to peek eye on both,so you need to switch to one another one by one,
If you can make a windows transparent with just hot key then you can see what’s going on the other one.
Peek Through is an application for Windows XP, Vista and 7 which makes the foreground window transparent with the press of Hot Keys. You may adjust the amount of transparency and the Hot Keys.
As you can see in the above image I had placed Explorer window above the Gmail homepage and you can see through the explorer window.
By default Win + A is the hotkey but you can change it by opening Peek through from your system tray.
Just select the desired the your choice of key and click on Activate.
You can also change the Transparency level by clicking on Transparency tab and moving the slider left or right according to your choice.
You need to have Microsoft .NET Framework 3.5 SP1 Or more
You may use following link to download it :-