Linux has a lot of eye-candy because of Compiz, my favorite of which is the window opacity plugin. Using a short AutoHotKey script, you can add that same functionality to Windows 7.

I used this AHK script as a basis for changing opacity. It uses a single hotkey to change the active window’s transparency by 25% each time until it resets to 0. I wanted functionality similar to Compiz, so I modified the script to use the mouse wheel and shortened the increments to get more variety.

Just hold down the Windows key and scroll down to see through the window. This decreases opacity and makes windows more transparent.