Simple text files, in markdown format. Currently working on a few little tools to manage them, including a simple web-based editor. That will let me take notes on Mac, iPad, Android phone, etc.
I have simply a folder that contains 10+ text files, mostly containing numbered lists:
01_calendar; 02_TODO (just a stack of things I want to do); 03_questions (e.g.: "What substances emit sound when heated?"); 04_ideas (entrepreneurship, organization -- this is helpful because for one thing you start to see the pattern in your ideas -- what you value etc.); 05_opinions&thoughts; 06_self&improvement (my habits are like this and that, I could improve them like this); 07_read&watch&experience (list of interesting people, movies etc.); 08_my_history (contains critical events that have happened to me, it's really useful to see them all at once for self-reflection); 09_principles_I_follow; 10_humor (humorous ideas that I collect in order to create a stand-up routine someday)
Nobody else mentioned it which really surprises me but I use Google Keep. Allows you to organise cards, colour code then, pin them and share them across devices. Anyone else use this? I find it's simple and restricted UI reduces distraction; just you and the idea. Also good for check lists, links and reminders.
You can also share Keep cards with other people's Gmail accounts.
I email myself and capture in emacs org-mode. I have an email address that is bound to AWS SES. SES listens for incoming emails and writes them to an S3 bucket. I then have a lambda hook which listens for writes to this bucket and processes the email content. This usually means writing an emacs org-mode record on dropbox. Which is then added to my emacs agenda.
I just e-mail myself, make sure to include “idea” or “business idea” or “game idea” or whatever in the subject line, respond to the email chain if i want to iterate, and periodically go back through the search results of emails I’ve sent myself.
I use Dynalist for everything these days - inbox, task, planning, notes, long-term storage of important information. So much, that I'm actually worried that I do not have full control over my data (I have backups at least, which is nice).