Closure Compiler is a JavaScript optimizer that compiles web apps down into compact, high-performance JavaScript code.

Closure Library is a broad, well-tested, modular, and cross-browser JavaScript library. Web developers can pull just what they need from a wide set of reusable UI widgets and controls, as well as lower-level utilities for the DOM, server communication, animation, data structures, unit testing, rich-text editing, and much, much more. (Seriously. Check the docs.)

Closure Templates grew out of a desire for web templates that are precompiled to efficient JavaScript.  Closure Templates have a simple syntax that is natural for programmers.