FireBug

Every developer "carries" his bag of tools around when doing their work...This bag of tools can represent a precious advantage since a good tool applied at the right moment can bring speed and quality to the development effort. It is quite unlikely you will find a tool that "fits all" (a tool for each job) and the developer does need to know enough of the tool and problem to solve to make it useful (it is not the weapon that makes the warrior, it is the warrior that makes the weapon). But sure enough, some tools are just too good and should be in every developer's bag. A tool which I found to be a must for any serious web developer doing from just regular HTML coding to AJAX development is the Firebug Firefox extension (you are using Firefox right?!?! ;-) ). The tool allows you (among other functionality) to:

• Inspect HTML, by moving your mouse around you can get to see the HTML snippet relevant to the element highlighted on the browser.
• Inspect properties like positioning, styles on inspected HTML elements.
• Inspect events happening in your pages.
• Inspect the DOM of your page.
  
• Perform Javascript debugging.
• Show all AJAX interactions between your page and the underlying server.
• Show all kinds of errors (e.g. CSS, Javascript, etc)

In short, if you are doing web development and have not seen Firebug in action before, you should spend sometime looking at it...It will save you hours in any future development. It sure is in my bag of tools to stay. :-)