Markdown is the lingua franca of open source, but until Canvas, none of the online collaboration tools we used understood it. Not only does Canvas support Markdown, it supports programmer-flavored Markdown, with great support for things like checkboxes and code fencing. It's become an indispensable part of how we build open source software online.
Just a few of the ways we use Canvas:
1. Writing feature proposals/documentation side by side with someone
2. Taking collaborative meeting minutes during the weekly core team Google Hangout
3. 12+ of us sitting around a couch, writing an agenda for our quarterly in-person meetings
Canvas is all about small details that continue to delight as you learn it. I think my favorite "hidden" feature is that you can add `.md` to the end of any document URL and it serves up a static Markdown file, making data export incredibly easy.