I've used all kinds of productivity apps but the best I've used (8 years and going) was a Google Sheet that I maintained with my friends where we would keep our to-do lists. Being able to see each others lists would make us actually complete those tasks in the face of peer pressure and shame. So we decided to make our sheet into a web app so that everyone can use it. It's basically a calendar where you keep a to-do list and your friends can view them. You can also comment on those tasks (it's great for keeping up with friends if you don't use Instagram like me) and get notifications. It's completely free, lmk what you think!