For accessibility spot checks and HTML validation as well as a full range of accessibility checking for Firefox 2.0 users - both Mac and Win:
Web Developer Extension
http://chrispederick.com/work/webdeveloper/
Fangs (text based screen reader emulator)
http://www.standards-schmandards.com/projects/fangs
The University of Illinois at Urbana/Champaign Mozilla/Firefox Accessibility Extension
http://firefox.cita.uiuc.edu/