WebAIM recently released the WAVE Toolbar for Firefox, which augments their free online accessbility evaluation service, known as WAVE. WAVE is currently in version 4, much like Cynthia Says but with a focus on developer friendliness by overlaying results visually above the page you are testing. I think WAVE is a pretty cool idea. The fact of the matter that no automated tool can ever tell whether a website is truly accessible means it always boils down to human assessment in the end. So a visual tool is much welcomed as the explanations you get from Cynthia are quite cryptic at times.
For more power in your toolbar I recommend trying out the Firefox Accessibility Extension. This one is primarily an assistive tool for people with disabilities browsing the Web but also doubles up as a great accessibility evaluation tool for developers.