Window-List is a simple App Indicator (i.e. an app that lives in the notification area) that displays a list of all open windows at any given time.

The application provides its users with an easy way to switch between currently open windows. To access a window, simply click on its icon and you will  be directed to it immediately.

Ubuntu users can easily install Window-List using the following PPA:

sudo add-apt-repository ppa:jwigley/window-list
sudo apt-get update
sudo apt-get install window-list