By WebAIM, available for Chrome, Firefox, and Microsoft Edge.

Checks single pages. Availble for Chrome, Firefox, Edge, and Opera.

by Deque, Chrome extension, must be used in “Inspect” mode.

By Microsoft, extension includes a guide for manual assessments.

A "favelet" that will provide automated detection of accessibility issues

By WebAIM, includes lightness sliders to adjust the colors to ones with sufficient contrast.