FireBug ( http://www.getfirebug.com/ ) is probably the most valuable tool I've found. Points out errors in javascript and CSS when you browse to a page, has an html DOM that shows all the styling that gets applied to each node and where it comes from, and a report showing all the hits involved in rendering the page, and how long each of them took to retrieve.

IE Tab ( https://addons.mozilla.org/firefox/1419/ ) looks really interesting; it lets you run IE in a tab inside FireFox. Very cool for cross browser checking, and also for me to check my web stats, since I copy some of the data into a spreadsheet, and FF won't do this right.

SEO Quake ( http://www.seoquake.com/index-en.php ) looks really interesting, although Google's terms of service prohibit automated queries to analyze by software, or at least so I thought.

What else is worth looking into?

1. Anything with IE sucks green donkey's IMO.

2. SEO Quake is a joke.

3. Firebug was great when it first came out, so you might want to try something far more powerful that many of us Webmasters have been screaming to see the final release and it's called...Web Developer Toolbar

Well, my recommendations - Web Developer Tool Bar and Firebug. I don't use the IE tab, but I do use an extension that will view a page in IE, I just right-click and 'view this page in IE', it's very handy for testing and there's no question it's 100% IE with all its bugs.

I also use the Fangs viewer and the Lynx viewer for accessibility.

faster fox is pretty cool too! noticed a slightly faster loading with pages and webmaster toolbar is great!

ColorZilla is great too for web designer

I use firebug, web developer tool bar, colorzilla and measureit. Measureit has been great to measure designs for a more exact layout translation from photoshop. I use the Web Developer Toolbar every day, most especially the DOM Inspector and Information tools.

I'm going to have to check out the IE plugins, that would be helpful. Which ones are best? LadynRed, I'm curious what you're using. I'm on a Mac so this sounds expecially helpful. It would keep me from traveling back and forth between computers to test.

I've just this second installed that SEO Quake thing... well I have just found a couple of people have bookmarked one of my videos on my site, before the layout change. Hmmm.. interesting little gadget, though annoying a tad.