WAVE (Web Accessibility Evaluation Tool) by WebAIM is a tool to help web developers make their web content more accessible. WAVE cannot tell you if your web content is accessible. Only a human can determine true accessibility. But, WAVE can help you evaluate the accessibility of your web content.
WAVE is a browser plug-in for Chrome and Firefox. View the WAVE help page for more help.
Siteimprove is both a robust web-site analysis tool that checks both accessibility issues and quality assurance as well as a stand-alone web page checker.
The Colour Contrast Analyser (CCA) by The Paciello Group helps you determine the legibility of text and the contrast of visual elements, such as graphical controls and visual indicators. With eye dropper color pickers, you do not need to know the Hex value of the colors being checked.