wwww.cronote.com

Simple website for sending reminders. It didn't make any money for 4 years until I added commonly-requested features (templates, recurring reminders, and variables for recipient names). Now I have about 60 paying customers and it makes about $600 per month after all expenses paid. It's been running essentially autonomously for the past five months.

The iOS app is still difficult to use. I'm slowly rebuilding it. I'd say the main claim to fame at this point is if you Google "text message reminders" it's the first or second result.