Web Developer - An add-on that allows you to highlight sections of the page, display alternate text and quick links to validation sites.
Firebug - An add-on that allows you to inspect elements. I find this better than the inspection tool built into recent Firefox versions. It also includes a JavaScript debugger for those that want to dive into JavaScript development.
Fangs - Experience your web page in a way that a screen reader would describe for those with disabilities
Accessibility
Introduction to Web Accessibility - A great introduction to what we must do as part of making our websites accessible as part of State and Federal Law. The entire site has much more about this topic.
Useful Tools
W3C HTML/XHTML Validator - Make sure that your pages do not have errors and they will be easier to maintain.
Web Page Analyzer - Shows how fast a given page will load and will let you know when the number of images and other page content gets too high before people get frustrated with your site.
WAVE - Web Accessibility Evaluation Tool. Inspects pages for potential accessibility problems. See Using WAVE