Last month, I gave some links for validating your HTML. Here are some links to help test the accessibility of your pages.
Bobby http://webxact.watchfire.com/ checks the accessibility of your pages for people with disabilities. For certain kinds of sites including many governmental and educational ones, this is mandatory, not just optional.
http://www.webable.com/ has lots of links and resources to accessibility information and they also have an Accessibility Monitor that will check up to five URLs for you for free. http://www.accessibilitymonitor.com/
It's not really possible to discuss 'accessibility' without a nod in the direction of its arch-advocate, Jakob Nielsen.
Nielsen's site contains a lot of good information, although I know many designers who think he is a bit extreme in his views. His site frequently analyses other major sites for their accessibility, or lack of it. Where I would agree with most of what he says, there is also a broader picture.
Accessibility IS a very important factor to be considered when designing a website - but it's not the only one.