SSL certificate checker
See when a certificate expires, who issued it, and how long you have. No signup, no limits worth mentioning.
Why certificates lapse
Almost never because someone decided not to renew. They lapse because the person who set up auto-renewal left, or the renewal cron stopped running months ago and nothing said so, or the certificate was issued outside the automated process and nobody remembered.
The failure is loud when it happens — every visitor sees a full-page browser warning — and completely silent up until then. That gap is the whole problem.