Cronote made next to no money for 5 years, but slowly migrated its way to the top of Google for search of "text message reminders" and "sms reminders." I did a ton of work, made an iPhone app, changed the billing system four times and rebuilt the interface three times.
I now have ~60 paying customers, but it only makes $300 per month after all expenses. I don't think I've recouped the man-hours spent, but I've certainly learned a lot along the way. I learned Python/Django (website back-end), Objective-C and Swift (iOS app), and lots of fun stuff about timezones and calendaring APIs.