While not exactly software per-se, I created a system of multiple text files to manage todos, long term goals, and various reminders (eg, IOUs, deadlines, etc). This was inspired initially by Jeff Huang’s blog post [1] but then grew to a complex collection of different files. A problem I ran into was building an interface for displaying and editing these text files (each file has a different width and for some files I want to have different heights when editing them). Ultimately I settled on multiple vim tabs in a terminal window. Been using this for close to five years now and I couldn’t be happier with it. However, at this point the system of files (and the terminal “user interface”) is completely customized to my life and would likely never fit someone else’s requirements.

[1] https://jeffhuang.com/productivity_text_file/