Firebug is a really cool extension to Firefox that one of our designers showed me yesterday. It inspects the current page and displays all sorts of great info about it. That's okay and there have been other scriptlets that do similar things. Where it goes beyond others is that you can actually edit anything and have it show up on the page, whether you own the page or not. Obviously, it is not saved, but it's a great way to experiment as you are building your pages. In the picture below, I changed the title text in the HTML output and the color CSS of this blog.
You can also modify scripts, debug, and inspect the size and download speed of the page (and how long each request took).
Check it out. I just started playing with it, so I'm sure there is more that it can do.
Manager, MA Design Studio
IBM Lotus Software
Chris Reckling | 9 May 2007 09:42:54 AM ET | Westford, MA