We built this for our internal use, but decided to make it our community project. It is highly scalable, free, and will always be free. It is a globally distributed monitoring service that checks for missing, expiring or misconfigured certs and alerts multiple recipients via SMS or email. This is intended to be a third-party supplement to the email expiration notifications LetsEncrypt already sends out.
With 90 day expirations, it can get a little tricky validating all your certs are up to date and functioning.