Google Keep

For sticky-note thinkers who jot a grocery list, a parking spot and a half-formed idea within the same minute. Keep's colored cards, labels and reminders sync across Android, iPhone and the web through your Google account, and it is free, so it makes an excellent inbox even if your real notes live somewhere else. Leaving is the untidy part. The exit is Google Takeout, which hands you a ZIP with each note as both an HTML file and a JSON file. Plenty of apps can import that, but nobody would call it a pleasant format to own.