Colour Contrast Analyser - TPGi's ADA Color Contrast Checker checks for helps determine the accessibility the contrast ratio of two colors for WCAG AA and AAA.
accessiBe - Making websites accessible to people with disabilities
Who Can Use - WhoCanUse is a tool that brings attention and understanding to how color contrast can affect different people with visual impairments.
Page Speaker - Modern web accessibility tools for agencies & webmasters
Kontrast - Check and adjust web element contrast in real time.
Stark - Contrast check and color blindness simulator for Sketch.