Self-hosted alternative to Google Doc's collaborative features?

Generally, when I'm writing something I like to send it out to a few of my friends and such to get feedback and discussion. Thus far the best tool I've found for this is Google Doc's comments. I've sort of come to realize this is the last step in my writing process that I don't have very much control over. I can edit drafts from pretty much anywhere, "publishing" is just a self-hosted website.

I would switch to code review style tools, but none of the people I send these drafts to are technical in any way, so things like GitHub are out. We're sort of remote, so old-school paper drafts are out (as much fun as that would be). Email threading would probably be a good option, but I find it super unwieldy after one or two comments.

Are there any good options? What do you folks use?