I just saw a session (something about secrets of JavaScript libraries) with people from Dojo, JQuery, Prototype.

there are a few random tools they have mentioned.

1) detecting memory leaks in Internet explorer - Drip : http://sourceforge.net/projects/ieleak 

2) shrinking javascript (removing whitespace/comments etc) - ShinkSafe : http://shrinksafe.dojotoolkit.org/