CynthiaSays is a web-based compliance checker which checks a given URL for accessibility.
After the user enters the URL of a publicly-viewable website and selects Section 508 or WCAG 2.0, the software will return a report of accessibility issues at that URL, showing either a green check mark for items that are handled in an accessible manner, a red x for those that are not, a grey minus for items which do not apply (e.g. if a page has no HTML tables, the tool will not be able to tell if your tables are accessible), and a blue eye for items that require manual attention. Users checking against WCAG 2.0 can select from the three different levels of compliance defined by the WCAG guidelines.
Although CynthiaSays offers limited guidance to users, one useful touch is that each item in the results is a link to the relevant section of the WCAG guidelines or to relevant resources on Section 508, so users can find more information on why this item is a problem and how to resolve it.
The tool is a joint project of Cryptzone, ICDRI, and the Internet Society Disability and Special Needs Chapter. You can test it out or learn more about it here: http://www.cynthiasays.com/
Note that CynthiaSays should not be the sole method of determining accessibility, and is intended for personal, non-commercial use only. However, it can still be an excellent way to find some common accessibility issues and learn about possible solutions.