We have this at our website monitoring and recovery service https://t1mr.com and it's free.
Also, if you check SSL cert validity, you should totally advertise that! I remember I've been paying for one such service where they would monitor SSL url, but would not alert you about SSL cert expiration. I found out the hard way.
The advanced (paid) functionality is a restart of your vps or other automatic ssh action trying to recover it when it's down.
[Errno 1] _ssl.c:504: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed