Thanks, we've built it for ourselves and wanted to see if anyone else would be interested first. Would happily add accounts and charge a small subscription fee for it. How many dollars per month would it be worth to you?)
What's unique about what you have here is the realtime collaborative editing. We use NestaCMS (which also understands Markdown) hooked up to a Dropbox folder in a few places. The functionality is similar, but there's no in-page editor, and we can't collaborate in real time.
Both of those things have value to me, but I have serious reservations about jumping on to a wiki product that I can't control. Documentation is a complete pain in the ass. When I put time in to it, I need to feel 100% confident that I won't have to abandon it at a future date; either because I run up against a feature ceiling, or because you disappear. And if I do have to leave, I need to be able to get my information out of the system.