That said, I wouldn't pay for a service like this from a random person, I'd have my registrar do it (MarkMonitor or similar -- that's why they're paid the big bucks).
In an old job for a mid-level bank, we had many behind the scenes (never showed up as a green padlock in any browser) integrations that would quit working when certs expired, frequently ones that we didn't purchase or control.
At the time, it was definitely worth it to me to get some notice before that happened. And it can be easier to plonk down a credit card than work though internal processes to have nagios (or patrol express, <shudder>) do the monitoring. It's the same thing that makes pingdom valuable.
Can I ask why you wouldn't pay a "random person" as you say - since this information is by definition public?
Can you tell me a bit about your experience using MarkMonitor to do this?
For my personal set of servers (some 25-30, with ~50 SSL certs), I have Nagios for monitoring them plus calendar alerts, SMS alerts and sane cycling (everything expires in the same month).
My employer is an intermediary CA, they can issue their own certs but I've worked with people who use Markmonitor. As part of buying your cert through them is they worry about making sure your domains stay protected. They'll call you, text you and even mail you reminders. And they have a proven track record dealing with companies that are collective worth trillions.
