Nagios plugin check_http has a -C option that allows you to be warned when the SSL certificate is about to expire. It always annoyed me, however, that the expiry date was printed in crazy-ass US date format (which doesn't make any sense).
I've altered the source code so that it prints the expiry date in human readable format.
The changes were made to file plugins_sslutils.c. You can download a patch for version 1.4.10 here. Or just download the changed file; it might work for other versions too.