overstimulate

Ten good practices for writing JavaScript in 2005 has a lot of good information for hacking Javscript. Although, he missed my favorite tool, Javascript Shell

They also offer it as a bookmarklet to add to your bookmark bar. You are never more than a click away from using javascript inside the context of the current page. It’s great for testing ideas for GreaseMonkey scripts, ajax, effects or playing with the DOM.

(The bookmarklet is for Mozilla based clients only, but the none bookmarklet works in most others)