Web/iOS: Microsoft Office and Google Docs are the two main powerhouses in the collaborative documents world, but if you're looking for a bit of different approach, Canvas is a tool that allows you to write in Markdown, and easily collaborate with anyone. Markdown is a nice and easy way to take notes quickly without the need for toolbars and formatting dropdown menus. Canvas simply takes that idea and adds a collaborative layer to it. You can share a link to your document with anyone, and they can pop in to look at it, make edits and add notes without even needing an account. It's basically a slightly simpler version of Google Docs, which alone makes it a nice tool for getting notes down and shared quickly.