Since recently my work is primarily related to Web Application Performance Tuning, some tools are needed to work efficiently.
Talking about web debugging / performance tuning tools, at Microsoft side the "Fiddler" tool should be a must one. recently it just released 126.96.36.199 version at 2007/04/12, which is as same as the "Fiddler version 2" group originally from 2006/08/09.
Fiddler and new Fiddler 2 can be installed side-by-side in the same box without problems, which Fiddler 2 provides HTTPS traffic debugging abilities. also the scripting ability is a plus for customizing debugging web applications.
While installing Fiddler, also noticed that IE Developer Toolbar seems also got new update at 2007/05/09 (version 1.00.2188.0), which is also a nice tool for debugging HTML DOM documents.
Since there is plug-in programming model existed in FireFox, there are many good tools to use for debugging web applications. what I am using now are:
you can find more web debug tools at this post (in Japanese).