CoffeeScript is a language written by Jeremy Ashkenas that compiles to Javascript. Its syntax is much more friendly than native JavaScript. Especially if you're used to languages like Ruby or Python.

As or Rails 3.1, CoffeeScript is available by default and a coffeescript file is created in /app/assets/javascripts every time you run the scaffold generator.

A few of my favorite features of the language are:

I can ignore parenthesis (mostly), semi-colons, and curly braces

Function notation

Objects with nested notation

Variable safety (not creating globals)

Splats on lists

Classes

Always returns last expression

String interpolation

I don't like:

Operator aliases

Resources:

CoffeeScript Webpage

CoffeeScript Cookbook

CoffeeScript Basics (video)

CoffeeScript: The Cool Parts (video)