Cert Canary by David Tucci

Cert Canary is a browser extension designed to warn if a TLS certificate is near expiration.

A yellow warning box will appear if a page results in a request served with a certificate due to expire within 30 days. If there are fewer than 15 days until the certificate expires the warning box will be red.

Clicking on the canary icon will show a list of domains the page has requested, color coded based on the time to their certificate expiration.