Gecko

Gecko is the name of the layout engine developed by the Mozilla Project. It was originally named NGLayout.

Gecko's function is to read web content, such as HTML, CSS, XUL, and JavaScript, and render it on user's screen or print it. In XUL-based applications Gecko is used to render the application's user interface as well.

Gecko is used in many applications, including a few browsers, such as Firefox, Mozilla Suite, Camino, etc. (for a complete list, please refer to Wikipedia's article on Gecko.)