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/
PingBack from http://msdnrss.thecoderblogs.com/2008/03/11/tool-detecting-memory-leaks-in-web-ie-shrinking-javascript/
Nice one, another one that's used often is YUI's compressor - see http://developer.yahoo.com/yui/compressor/