The best web page development tool there is.

If you create web pages and you haven't heard about this tool yet, you need to get it ASAP. It will make your life so much easier. How I went this long without hearing about this is a mystery to me, but I guess you can't know everything. The tool is called Firebug (http://www.getfirebug.com), and it's a toolbar for Firefox that allows you to make live modifications to CSS and see the changes instantly in the browser. Also, the JavaScript debugging is spectacular (by JavaScript standards). Inspect mode has been a life-saver for me - it lets you click on an element in the web page, and takes you to the relevant HTML. Other tools, like the IE Developer Toolbar, have some of the features - but nothing else has them all.

Sorry if this is old news to you, but if you haven't tried it yet, you should. I wouldn't want to be without it.