The WAVE Toolbar provides button options and a menu that will modify the current web page to reveal the underlying accessibility information. It provides an easy-to-use, yet powerful mechanism for evaluating the accessibility and usability of web content for people with disabilities.
The currently rendered document is evaluated directly within Firefox. This allows accessibility evaluation of CSS, scripted, AJAX, DHTML, dynamically generated, and secure web content
Over 80 possible icons show missing alternative text for images, missing form labels, table structure, script elements and event handlers, document structure and reading order, and much more.
More details and a server-based version are available at http://wave.webaim.org/