One drawback is that it only works with single pages, and doesn't yet have the ability to spider entire sites. It's currently available in beta, and the A-Prompt folks are working with developers to incorporate the tool into future releases of major web editor and courseware products. Its distinguishing feature is its accessibility wizard, which prompts developers with specific recommended fixes. Insight is a web accessibility assessment tool. Infocus, like A-Prompt, provides an interface through which the web developer can correct accessibility problems.
Clarity generates reports on the accessibility of an entire web site, including summary statistics.
Unlike the other three products described above, these are commercial products, and many consider them to be highly priced. The following links provide additional information, particularly regarding the accessibility of emerging technologies, and the accessibility of web-delivered science, math and engineering curricula. Additional information is on the LTS Assuring Accessibility web site.