fn + f10 or just f10 is the default (depending on your keyboard settings). You can set it in System Preferences > Exposé and Spaces, it is the "Application windows" option.
I don't believe that you can set it to be activated by middle clicking, though. The fastest way to do it would probably be command + tab to navigate to the application that you want and then fn + f10 to select the window that you want. Although that is only if the application is not active at the time.