Quick call for Mou 1.0 fundraiser on Indiegogo. At the time of writing, it has raised $6,178/$20,000, and has 39 days to go (with 21 already passed).
I'm actually writing this post in Mou right now. It's far less powerful than Emacs, but when I want preview-on-the-fly, Mou is the Markdown editor to go. Right now it's far from perfect; for instance, GFM fenced blocks (now included in CommonMark) are not supported, so you get nonsense preview when your code block is fenced rather than indented. (Of course, Mou is even less suitable for editing an Octopress post due to the yaml metadata upfront, but that's not a big deal.)
Let's hope for Mou hitting the 1.0 mark.