Tanaguru Contrast Finder is a tool to check the contrast ratio between two colors. When two colors don't meet the desired contrast ratio, it helps you find a new color that can solve this problem.

Color Review shows the contrast ratio between two colors. It has a feature that previews sample content of what the combination would look like.

Contrast ratio is a tool which lets the user pick a color for the text and a color for the background, and checks the given text against the background

Color Contrast Checker From Polyplane is a tool that suggest colors that meet a desired contrast ratio.