Interesting new collaborative document editing tool / service from the ex-CTO of Facebook. Labeled as a modern word processor that enables you to create beautiful documents on any device, including the iPad, iPhone, Android, and the desktop.
Think of Google Docs but with better collaboration, discussion and user interface. I've had a quick look and there is a lot of potential here. I'll be taking it out for a longer spin over the weekend.
In the same vein, what would be ideal and keeping in the spirit of having your data open would be to build a collaborative document editing tool around git and markdown, wrapped in a nice user interface. A potential downside I see to apps like Quip is that you're somewhat data-locked.