Closure Tools is set of opensource (release info) developer tools by Google which are used while building Gmail, Google Maps, Google Docs & many others.
The compiler also offers a Firebug extension, named Inspector, for better debugging.
Only what you need can be pulled from a large set of reusable UI widgets, controls, lower-level utilities for DOM manipulation, server communication, animation, data structures, unit testing, rich-text editing, & more.
They simplifty the task of dynamically generating HTML.
In contrast to traditional templating systems, in which you use one big template per page, you can think of Closure Templates as small components that you compose to form your user interface.