CoffeeScript is a little language that compiles into Javascript. Underneath all of those embarrassing braces and semicolons, Javascript has always had a gogeous object model at its heart. CoffeeScript is an attempt to expose the good parts of Javascript in a simple way.
The golden rule of CoffeeScript is: "It's just Javascript". The code compiles one-to-one into the equivalent JS, and there is no interpretation at runtime. You can use any existing Javascript library seamlessly (and vice-versa). The compiled output is readable and pretty-printed, passes through Javascript Lint without warnings, and runs in every Javascript implementation.
http://jashkenas.github.com/coffee-script/