The Javascript Specification

You can download the latest specification here (warning: it's a 2.93MB PDF; there's no HTML version yet, but the ECMA has someone working on it). It's labelled draft, but it was approved late this year and defines ECMAScript 5th edition (there was no 4th edition).

The previous version (3rd edition) is available from this page (why the latest isn't linked from this page, I don't know).