I will second the parent comment. I feel that there's a large number of people who have needs for this kind of product, but haven't recognized it: the need is latent. Unlike active needs (the bingo card creator program, for example), people don't just go searching the internet for latent needs. The number of people who are actively searching for "appointment reminders" are probably less than 1% of the total number of people who have the need. That's why latent needs are very hard to harvest in online marketing. How are you going to tackle this problem?