* Markdown-ish in a text editor
* Google Doc
* Google Sheets
The first is for any quick notes I need to take or anything that might turn into a code checkin, confluence page, etc. The second is for anything I think I'll have to share and may want collaborators. The third is for almost everything else.
I feel I disagree with a lot of people in the rest of the comments. I think plain text in general is a major pain to deal with. I recall several years ago having to deal with CSV parsing and I was aghast at how complicated that is to get "correct". Give me highly structured data formats please. I'll take yaml, toml, JSON or even XML over CSV, TSV or plain text at pretty much every opportunity.