When my wife was editing my books, she used Mou.app for live preview of the text so that she knows what the output is going to be like. The caveat was that Mou.app does plain Markdown and not Pandoc format which would mean the preview would be screwed up whenever there was a code block, etc., so, today morning, I hacked up an app called “Kalam” which does exactly that – live preview for Pandoc text.
The app is based on top of node-webkit (which I came across when I was wondering what Light Table is built upon), created by Roger Wang and others at Intel China Open Source centre, they’ve basically integrated node.js into webkit and disabled all the security restrictions, which makes it the almost-perfect cross-platform desktop toolkit – write HTML, CSS, JavaScript and use any node.js module!
Update: There’s also AppJS which is the same concept as node-webkit but looks more polished (via @aravindavk)