http://quip.com/ -- It works everywhere except, unfortunately, the terminal. What I like about quip's way is that it doesn't differentiate checklists from notes -- which is exactly my style. And you can even embed a spreadsheet if you suddenly need more power.

P.S.: They also don't like Firefox for some reason (it works ok, but it works better in Chrome). Now that Salesforce bought them, I don't have my hopes too high that it is going to change...