Thank you, but it's not a landing page, I just made this for this thread. I'll look for that. Do you know any startups in this industry?!
The biggest ones I'm aware of are AppointmentReminder.org, which is helmed by our very own Patio11, ReminderCall.com, and Inphonite. Beyond that, there are specific appointment reminder applications per given niche (there are like, 3 very large, very expensive ones geared towards health care appointments, for instance).
Do you know of a list of problems which have been solved ad nauseam? It would be useful when coming up with a 'revolutionary' new idea, to quickly check if it is :)
That said, I'm not aware of any such lists, though market research is always encouraged before spending time on development.