I thought I'd see Hawk Sugano's system mentioned here, but haven't so I'll share the link: http://pileofindexcards.org/

What's nice about his index card system is the simple taxonomy for organization, which can be expanded upon for specific projects or just carrying on day-to-day. Of course these can be modified, but I've found the four groups he proposes (Record, Discovery, GTD/to-do, Citation) work well for everyday capturing of ideas big and small.

This, and Marc Andreesen's note to make a short to-do list nightly for the next day are my main uses for index cards. https://pmarchive.com/guide_to_personal_productivity.html