Nautilus extensions are far more powerful than Nautilus scripts, allowing more freedom where and how they extend Nautilus. Nautilus extensions are typically installed by your system administrator.
Some popular Nautilus extensions include:
nautilus-actions
This extension allows you to easily assign actions based on file type
nautilus-send-to
This extension provides a simple way to send a file or folder to another using email, instant messaging, or Bluetooth.
nautilus-open-terminal.
This extension provides an easy way to open a terminal at the selected starting location.
If you are looking for the Open Terminal command which used to exist in the Nautilus right click menu by default then you should install the nautilus-open-terminal extension.