On Windows, I just use the Microsoft Sticky Notes app. I'd used text files for a long time, and tried various local and web-based apps, but the key feature for me is that the note has to be visible or I'll eventually forget about it. I've got a whiteboard beside my desk and that's good for some things, but having the notes on my desktop is easier to edit. My second monitor on each work computer is a mess of notes, but I never forget about them.

https://en.wikipedia.org/wiki/Sticky_Notes