Like every programmer before me, I created a to-do list app of course! However, as a programmer, I prefer trees to lists so I made a to-do tree app. I have been using it religiously every day since to manage all aspects of my life in a way that I couldn't previously with tools like Wunderlist (RIP) or didn't have the time to make work in this way like Notion.
Shockingly there aren't many other options out there for tree-based to-dos so when I posted it to my personal Twitter some people really liked it and started paying for it which turned my weekend project into more of a full-time side project.
As of today, I've been working on it for almost a year on and off and in fact yesterday I submitted my first iOS app to the app store for approval (a quick rejection but easily solvable!).
I think everyone should build their own to-do list app not least because it encourages you to actually use a to-do list app but also because I've learned so many interesting lessons from it along the way. Through this project, I've learned Svelte, iOS development with SwiftUI, lots of things about PWAs, and much more that is not easy to convey about engineering solutions.
You can find it at https://tatask.com if you're interested.