This extension aims to make Gnome Shell easier to navigate using only the keyboard. It is inspired by the vim plugins of Firefox and Chrome.
In the overview, you can press SPACE and letters are going to pop in the corner of every window. The search will be disabled and the user will be able to focus or close windows.
When the activation is done, to focus a particular window, you have to press its letters one after the other.
It is also possible to close a window in the same way, but while keeping SHIFT pressed.
A similar extension can be found here