A JavaScript optimizer
The Closure Compiler compiles JavaScript into compact, high-performance code. The compiler removes dead code and rewrites and minimizes what's left so that it downloads and runs quickly. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. These checks and optimizations help you write apps that are less buggy and easier to maintain.
A templating system for Java & JavaScript
Closure Templates simplify the task of dynamically generating HTML. They have a simple syntax that is natural for programmers. 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.
Closure Templates are implemented for both JavaScript and Java, so that you can use the same templates on both the server and client side. For the client side, Closure Templates are precompiled into efficient JavaScript.