Contrast Wizard
This tool tests 2 or 3 colors for contrast, and recommends colors with better contrast.
Color Contrast
APCA Readability Criterion • Contrast
WebAIM: Contrast Checker Bookmarklet
WCAG Color contrast checker
WebAIM: Link Contrast Checker
WCAG Contrast checker – Get this Extension for 🦊 Firefox (en-US)
Color Contrast Analyzer
WCAG Color contrast checker - Microsoft Edge Addons