Traceur
Traceur is a JavaScript.next-to-JavaScript-of-today compiler
...Traceur allows you to try out new and proposed language features today, helping you say what you mean in your code while informing the standards process. JavaScript's evolution needs your input. Try out the new language features. Tell us how they work for you and what's still causing you to use more boilerplate and "design patterns" than you prefer. You can use some language features right now and even try it out in your browser here. Just type in some code and see what Traceur produces.