For me it’s reminders. I use slack for work, and whenever anything comes up that I need to take care of (personal or work related) I’ll tell slack “/remind me about <thing> tomorrow/next week”.
I’ve been doing this for years. I have it on my phone and computer so it’s easy to view and add new reminders whenever I need. I didn’t think slack is crucial to use but I find it to be the most convenient, even more so then dedicated reminder apps