firefox as part of this balanced web breakfast
Do you do spend any time at all working on HTML or CSS? Maybe you just tweak your blog templates, or possibly you've taken up with the web standards camp (like I have)?
Whatever your ambitions, if you're looking to really speed up the feedback loop when testing your creations in a browser, you must run, not walk, over to
Web Developer Extension on chrispederick.com and download this extension for Firefox 1.0. (It goes almost without saying that you should already have Firefox installed on your machine, especially if you use Windows.) This extension allows you to X-Ray your HTML and CSS on live web pages and see where your layout and/or code is flawed. Plus, built-in hooks to popular code validation services further help trim unruly cruft from your code. If you're still just View Source-ing and reading those tea leaves, you're wasting a lot of otherwise productive time.
Browsers may still be forgiving of sloppy code, which is good, but if you're looking to achieve a certain layout and aesthetic goal, you'll do much better to play by the standards and create valid markup.