If you're doing web development or design of any sort, you have to check out Joe Hewitt's talk on Firebug at Yahoo! - download the movie or watch it online, just make sure you watch it.

If you aren't already using Firebug, this will convince you. If you are, you'll probably find some gems in there that you didn't know before. I didn't know that you could log events to elements on the fly, or press the up and down arrow keys to adjust numerical values (such as margins and paddings), or edit the values in the box model layout diagram, or hover over variable names to see a tooltip of their values in the JavaScript debugger, or conditional breakpoints.

Now if only it were able to detect JavaScript syntax errors that cause an entire JS file to be ignored by the browser.