Fangs for Firefox 3 is available:

http://www.standards-schmandards.com/2008/fangs-for-firefox-3/

It was great at presenting accessibility issues in a way that sighted individuals could immediately grasp – I even used it once to show a manager the accessibility implications of two alternative site designs. Unfortunately, the extension wasn’t updated to work with Firefox 2, let alone Firefox 3, and I miss it.

While learning about accessibility and extracting good practices from the Web Content Accessibility Guidelines, I found tools like Cynthia Says useful for quick checks. Beyond that, I use various features of the Web Developer Toolbar for testing accessibility. Here’s a useful article about that: Evaluating Web Sites for Accessibility with Firefox.

I also use the Web Accessibility Toolbar for IE for some tests, like colour contrast testing and sometimes for readability scoring.

You mentioned validation, Aaron. The HTML Validator extension for Firefox has an option to include accessibility warnings in its results based on WCAG checkpoints. I’ve also just discovered the Total Validator extension for Firefox which includes accessibility warnings too, but seems to be of limited value for daily use.

I think it’s important to say that it is about educating users as much as educating developers. There are extensions and plugins out there such as the Text size toolbar and the RNIB Surf Right toolbar that are helping to make useful accessibility features more prominent in the browser’s UI by putting it in the chrome rather than having them hidden away in menus and dialogs.

While it’s great that there is screen reader software freely available to test with (even JAWS and Window-Eyes have time-limited demo versions), I’ll have to disagree with slger about testing. Testing web pages with a screen reader as someone without decent knowledge of typical screen reader features and knowing enough about actual user behaviours can result in false positives and false negatives. I’ve been using screen reader software for some time (my sight is fine) and I know JAWS pretty well, but I do not consider myself to be a suitable replacement for user testing with disabled people.

NVDA, non-visual desktop access, from http://nvacess.org is free, open source. Could some of the test tools be integrated with this screen reader, e.g. to beep on the silly “click here” link text? NVDA, like all other screen readers, will have trouble with some web page features but will work sufficiently on most pages to reveal glaring errors. My favorites are: lack of H headings, redundant navigation links, and complex use cases. Compare the original and accessible Amazon pages to execute the simple use case to buy a book you already know you want.

Warning: while NVDA is trivial to install, testers might need to experiment with better voices available for about $30.

P.S. I am visually impaired, not a professional tester.