Ask HN: How do you keep track of your ideas?

I've never found a solution that I like for tracking and then later revisiting ideas.  I have Google Docs, spreadsheets, folders, and scraps of paper.  Are there web apps for idea tracking that you guys trust, and that hopefully allow both public and private ideas?  I'm also starting a Twitter feed for random ideas that I have: http://twitter.com/ideaperdiem