certmon
A simple certificate expiration monitor script.
It will connect to a given server on a given port, dump the certificate, and check if the certificate has expired or not.
Additionally, it will also report if a certificate is about to expire in x nr of days. (Default: 30 days).
On top of that, you can check the contents of certain fields in the certificate & see if it contains a keyword that should be there (to see if the certificate has been changed).
Reports will be sent via email, allowing you to schedule the script to run on regular intervals.