I’m a big fan of Brett Terptra’s app Marked for showing Markdown previews while writing in another app. Since I’ve been using BBEdit more for writing Markdown, I’ve wanted a way to preview the current document without leaving the app, so I took a few minutes to write an AppleScript which does that. If you haven’t previously saved the document, you’ll be prompted to. Once the document’s on disk it’ll open up in Marked.
Add it your
[!~/Library or ~/Dropbox]/Application Support/BBEdit/Scripts/ to launch it from the scripts menu in BBedit, or using script launching app like Alfred or Launchbar.