https://amazingmarvin.com/ has been my side project since 2016 (wow!). I will finally quit my job at the end of the year to go full-time on it (co-founder is already full-time). I worried a lot about doing something that had already been done 1000 times (a productivity tool), but now that it's working out I realized there's always room for a better mousetrap! Another surprising learning: it's still possible to have a product that people will pay for in 2019 with a mediocre phone app.