The JavaScript Kit have a pretty good one: http://www.javascriptkit.com/jsref/
A little harder to digest, but perhaps more complete (but only including the core, not web-specific stuff like window or document) is the ECMAScript specification: http://www.ecmascript.org/docs.php
And then there are the Mozilla and MSDN references, for browser specific stuff. https://developer.mozilla.org/en/JavaScript/Reference, http://msdn.microsoft.com/en-us/library/d1et7k7c(VS.94).aspx
