accessiBe - Making websites accessible to people with disabilities
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Sim Daltonism - From the perspective of a color blind person, some colors are impossible to distinguish.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Coblis - If you are not suffering from a color vision deficiency it is very hard to imagine how it looks...
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.